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.