What is the significance of Lord Rama seeking help from Sugriva? As Lord Rama is avatar of Lord Vishnu so I think he might know who abducted Sita Mata and where he kept secondly it was his personal matter so why did he involve Sugriva and his army when he alone is sufficient to destroy Ravana and all other demons? Why did Vanara Sena suddenly come into picture?
Rama took the help of Sugreeva in order to allow Sugreeva to fulfill his own promise to Rama and hence to save Sugreeva from the sins for making fake promises
You can find that Sugreeva promised to Rama that he will fetch Sita from the following slokhas
Then Sugreeva who is gladdened again spoke to Rama of Raghu's dynasty, "My best minister and this servant of yours Hanuma informed me of the reason for which you have come to uninhabited forests along with your brother Lakshmana, and that while living in the forest, and when you and this courageous Lakshmana are not her alongside a demon stole your wife Maithili, the daughter of Janaka, wailing as she was And he also said about that arch demon's killing an eagle named Jatayu on waiting for an opportunity, and his causing an agony in you by way of separating your wife. Before long you will get rid off your agony caused by the separation of wife, for I will fetch her very soon, like the retrieval Vedic Scriptures. Oh! Enemy destroyer, Rama, I will fetch your wife to give her to you whether she is in netherworlds or even in empyrean worlds...."
Although Rama can do the task alone without any help of Sugreeva, he just waited for allowing Sugreeva to fulfill his own promise to Rama. You can get it from the following words of Hanuman to Sugreeva
"Hence, the task of our friend is delayed, oh, enemy-destroyer, searching for Vaidehi is Raghava's mission and let it be done. Though the time is lagging, oh, king, that insightful and punctual Rama is not indicating about it to you, though he is hurrying to complete his mission within a time-frame, because he is following you alone abiding under the control abiding by your promise........... If need be Rama of Dasharatha is really capable of keeping gods, demons and great-vipers under his control with his arrows, but he is anticipating fulfilment of your promise"
If Sugreeva does not fulfill his promise, then he gets the sin of suicide along with the sin of killing his own kith and kin. In order to avoid it Rama took the help of Sugreeva by allowing him to fulfill his own promise. The following slokhas by Lakshmana to Sugreeva can clarify it
"A person gets attached to the sin of killing of a hundred of his manes, deified souls of his own ancestors, if he promises to gift one horse but fails to do so, and in promising to gift one cow but failing in it he gets the sin of killing a thousand manes, and if he promises to a person to do the needful help and fails in rendering that help, then he gets the sin of suicide along with the sin of killing his own kith and kin...."
While searching for Sita, Sri Rama and Lakshmana had an encounter with Kabandha. After incinerating him, he resurrects in a celestial form and advises Sri Rama to befriend Sugriva, who can cause search for Sita through his vanara army.
शृणु राघव तत्त्वेन यथा सीमाम् अवाप्स्यसि || ३-७२-७ राम षड् युक्तयो लोके याभिः सर्वम् विमृश्यते | परिमृष्टो दश अन्तेन दश आभागेन सेव्यते || ३-७२-८
"Oh, Raghava, how you will regain Seetha, that you listen from me in its essence... oh, Rama, by which and which analyses everything will be analysed, six of such ideations are available in this world... and when a person is touched down by a spell of nemeses, he shall adore one who is equally in such spell of nemesis... "
तत् अवश्यम् त्वया कार्यः स सुहृत् सुहृदाम् वर | अकृत्वा न हि ते सिद्धिम् अहम् पश्यामि चिन्तयन् || ३-७२-१०
"Oh, best among kind-hearted ones, thereby you have to certainly befriend such a soul in similar dire straits... however deeply I may think, I am not able to perceive your accomplishment if you do not befriended with such a soul."
यताम् राम वक्ष्यामि सुग्रीवो नाम वानरः | भ्रात्रा निरस्तः क्रुद्धेन वालिना शक्र सूनुना || ३-७२-११
"Oh, Rama, listen what I tell... a vanara is there by name Sugreeva, who is vengefully reneged by his brother Vali, the son of Indra."
स ते सहायो मित्रम् च सीतायाः परिमार्गणे | भविष्यति हि ते राम मा च शोके मनः कृधाः || ३-७२-१५
"Oh, Rama, he will be your associate and an aid in searching for Seetha, and he conducts himself in your wellbeing, hence let not your heart yield to agony."
Sage Valmiki described Sri Rama to be a human only, but not as incarnation of Vishnu. Sri Rama was undergoing a bad spell of time and he had to befriend another person undergoing similar bad spell of time.
It is an acceptable policy among kings.
First Sri Rama helped Sugriva and then Sugriva helped Sri Rama.
The main reason is that a Gandharva of the name of Viswavasu who became a Rakshasa because of a curse told that Sugriva will help him.
Thus addressed by Rama, that being replied unto him, saying, 'I am, O prince, a Gandharva of the name of Viswavasu! It was through the curse of a Brahmana that I had to assume the form and nature of a Rakshasa. As to thyself, O Rama, Sita hath been carried away with violence by king Ravana who dwelleth in Lanka. Repair thou unto Sugriva who will give thee his friendship. There, near enough to the peak of Rishyamuka is the lake known by the name of Pampa of sacred water and cranes. There dwelleth, with four of his counsellors, Sugriva, the brother of the monkey-king Vali decked with a garland of gold. Repairing unto him, inform of thy cause of sorrow. In plight very much like thy own, he will render thee assistance. This is all that we can say. Thou wilt, without doubt, see the daughter of Janaka!
Another reason can be this.
The vanaras were born to be the allies of Vishnu. Vishnu incarnated as Rama and the vanaras were the sons of the devas. Brahmarshis and others complained to Brahma about the same thing.
"Markandeya said, 'Then the Brahmarshis, the Siddhas and the Devarshis, with Havyavaha as their spokesman, sought the protection of Brahma. And Agni said, 'That powerful son of Visrava, the Ten-headed cannot be slain on account of thy boon! Endued with great might he oppresseth in every possible way the creatures of the earth. Protect us, therefore, O adorable one! There is none else save thee to protect us!' Brahma said that Vishnu is on the work to destroy him and that no god or asura can vanquish Ravana.
"Brahma said, 'O Agni, he cannot be conquered in battle by either the gods or the Asuras! I have already ordained that which is needful for that purpose. Indeed his death is near! Urged by me, the four-headed God hath already been incarnate for that object. Even Vishnu, that foremost of smiters will achieve that object!' Brahma then asked Indra and other devas to have bears and monkeys as their offsprings so that they will be the allies of Lord Vishnu.
"Markandeya continued, 'Then the Grandsire also asked Sakra, in their presence, 'Be thou, with all the celestials, born on earth! And beget ye on monkeys and bears, heroic sons possessed of great strength and capable of assuming any form at will as allies of Vishnu!'
The gods and danavas then formed a counsel as to how they should be born according to their respective parts.
And at this, the gods, the Gandharvas and the Danavas quickly assembled to take counsel as to how they should be born on earth according to their respective parts.
Brahma also told Dundhubi to be born as Manthara and cause quarrels.
And in their presence the boon-giving god commanded a Gandharvi, by name Dundubhi saying, 'Go there for accomplishing this object!' And Dundubhi hearing these words of the Grandsire was born in the world of men as the hunchbacked Manthara. And all the principal celestials, with Sakra and others begot offspring upon the wives of the foremost of monkeys and bears. And those sons equalled their sires in strength and fame. And they were capable of splitting mountain peaks and their weapons were stones and trees of the Sala and the Tala species.
Brahma instructed Manthara to cause quarrels.
And the adorable Creator of the Universe, having ordained all this, instructed Manthara as to what she would have to do. And Manthara quick as thought, understood all his words, and went hither and thither ever engaged in fomenting quarrels."
Sugriva is the son of Surya so we can say he was born to be one of the allies of Vishnu's avatar Rama.