It has been mentioned in the chapters that the knowledge of Savda Brahma and Para Brahma are absolutely necessary. At the same time it is needed to master the knowledge of rites and Sruti.
Lord Agni said that Lord Vishnu is the universal dissolution and he is the Rudra. The great Lord has the capacity to take a number of forms. The chapters have said that Lord Vishnu takes the form of a fish as well as tortoise and the two forms are known as Matsya and Varaha respectively. Agni in the Purana further elaboarted that the cause of all things is Brahma and that is the reason Lord Vishnu takes various forms like fish and tortoise because the cause of life or Prana in all of them is Brahma, the Truest of the true. This is because Vishnu is the cause of Creation.
It has been said that the reason behind various incarnations of Lord Vishnu was the protection of the pious and the destruction of the wicked. The story about the incarnation of Lord Vishnu in the form of fish narrates that the time when men and earth submerged under the rising ocean it was then the Matsya Avatar of Vishnu was seen by Manu for the first time.
When the fish went on extending its size depending on the size of its accommodation Manu was astonished and realised that the fish was none other than Vishnu. Manu had said that Vishnu was the one who had illusory power. It has been mentioned in these adhyayas that Lord Vishnu had incarnated himself in the form of fish for the protection of the universe and the destruction of the evil.
Agni says that Vishnu had incarnated in the form of boar or Varaha for the destruction of human sins. It has been said that Vishnu had had supported mount Mandara taking the form of a tortoise. Lord Vishnu taking the form of boar had protected the gods as well as their virtues. And soon after he had protected the gods from the asuras, he disappeared.
The adhyayas discussing about the incarnations of Lord Vishnu has said that taking the form of man-lion Vishnu had killed Hiranyakashyap together with all the Asuras and in turn had established the Suras (celestials) in their own stations. And that is the reason as to why the man-lion incarnation of Vishnu or Narasimha is worshipped by the celestials.
The Lord had taken the form of a dwarf in order to ensure the safety of the celestials. Parashu Rama incarnation of Lord Vishnu was in order to relieve the earth of her burden and for the sake of establishing peace.
Agni in his discourse had said that even Lord Buddhawas the incarnation of Vishnu too. It has been said that Lord Buddha was born to Suddhodhona in order to protect the gods as the gods had sought refuge to Iswara to protect themselves.
Finally Agni said that last avatar of Lord Vishnu in this universe will be Kalki Avatar who will come to this world in order to emancipate this world from all kinds of sins and bad deeds. Kalki will be the one who will establish the order and dignity in the universe once again. He will also show the path of religion to one and all. According to the verses of the ancient text after the Kalki avatar the lord will return to heaven for once and all.
Finally it can be concluded saying that the various incarnations of Lord Vishnu throughout the ages was mainly to protect mankind from all evils and to establish an order in the society.