Each time Lord take avatar it is in different form. Why not take avatar of same animal or same human each time? Is there any significance of incarnating as different form?
The answer is given in Shreemad Bhagvat Purana in Skanda 1 Chapter 3 , (The description of Avataras) this chapter is about twenty two avataras of Lord Vishnu. This chapter tells us why lord has took many forms in different avatara with explanation.
Form the description given in this chapter we come to conclusion that , lord took many forms like (Animal ,Siddha ,Yagna ,king , medicine god ,female ,amimal plus human etc.) in different avataras according to situation , need and purpose.
So in each avatara lord performed various different tasks that were suitable to that form he took.eg. it was necessary to take Narashimha avatara in half human half lion form , because of boon Hiranyakashipu received or it was necessary to take human form as Veda Vyasa to compile vedas , which simply were not suitable to perform other than human forms.Like Wise It was suitable to take tortoise form to to support Mandarachal mountain ,during churning of ocean.
Following shlokas mentions some of the forms and the purpose of lord to take that form -:
तत: सप्तम आकुत्यां रुचेर्यज्ञोsभ्य जायत।
स यामाद्ये: सुरगणैरपात्स्वायम्भुवान्तरम्॥१२॥
tataḥ saptama ākūtyāṁ rucer yajño ’bhyajāyata sa yāmādyaiḥ sura-gaṇair apāt svāyambhuvāntaram
Meaning-: The seventh incarnation was Yajña, the son of Prajāpati Ruci and his wife Ākūti. He controlled the period during the change of the Svāyambhuva Manu and was assisted by demigods such as His son Yāma.SB 1.3.12
ऋषिभिर्याचितो भेजे नवमं पार्थिवं वपु:।
दुग्धेमामोषधीर्विप्रास्तेनायं स उशत्तम: ॥१४॥
ṛṣibhir yācito bheje navamaṁ pārthivaṁ vapuḥ
dugdhemām oṣadhīr viprās tenāyaṁ sa uśattamaḥ
Meaning -: O brāhmaṇas, in the ninth incarnation, the Lord, prayed for by sages, accepted the body of a king [Pṛthu] who cultivated the land to yield various produce, and for that reason the earth was beautiful and attractive.SB 1.3.14
सुरासुराणामुदधिं मन्थतां मंदराचलं।
दध्रे कमठरूपेण पृष्ठ एकादशे विभु:॥१६॥
surāsurāṇām udadhiṁ mathnatāṁ mandarācalam
dadhre kamaṭha-rūpeṇa pṛṣṭha ekādaśe vibhuḥ
Meaning - The eleventh incarnation of the Lord took the form of a tortoise whose shell served as a pivot for the Mandarācala Hill, which was being used as a churning rod by the theists and atheists of the universe.SB 1.3.16
तत: सप्तदशे जात: सत्यवत्यां पराशरात।
चक्रे वेदतरो: शाखा दृष्ट्वा पुंसोsल्पमेधस:॥२१॥
tataḥ saptadaśe jātaḥ satyavatyāṁ parāśarāt
cakre veda-taroḥ śākhā dṛṣṭvā puṁso ’lpa-medhasaḥp
Meaning - Thereafter, in the seventeenth incarnation of Godhead, Śrī Vyāsadeva appeared in the womb of Satyavatī through Parāśara Muni, and he divided the one Veda into several branches and subbranches, seeing that the people in general were less intelligent.
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