The current work is dated to between the eighth and the eleventh century.
Many hymns are dedicated to Agni in the Vedas. Fire may keep the home warm and dry and cosy. As long as there is Fire (metabolism) in you, you live. "Not too little, not too much" applies. Besides, "Handle with care" to reap benefits from it is largely fit.
Tradition has it that Agni Purana was originally recited by the Lord of Fire, Agni, to Sage Vasishta, who told it to Vyasa, who told it to his disciple Suta, who told it to many sages who had gathered in the Naimisa forest once. The sages welcomed Suta, the reciter of ancient lore, to tell "the quintessence of all things, by knowing which one gets omniscience."
Suta told them: "The Supreme Being is the quintessence." (1.1-3)
He went on to tell how several animals had saved mankind earlier. A fish, a tortoise, a boar - and other Visnu descensions into forms.
On gods and antigods in Hinduism
In ancient times, the roles of different gods might have been different than what they are said to be today. Further, some gods became more prominent, and others receded toward the background of the scene.
In the earliest layer of Vedic texts Agni, Indra and other gods are also called Asuras, or "lords" of their respective domains, knowledge and abilities. In later Vedic and post-Vedic texts, usually benevolent gods are called Devas, while malevolent Asuras compete against these Devas and are considered "enemy of the gods".
For example, Book 1 of Rig Veda describes Savitr (Vedic solar deity) as an Asura who is a "kind leader" (1.35.10). In Rigvedic times, the Asuras were not yet demonised. Moreover, some of the well accepted gods in today's Hinduism, were asuras. Mitra and Varuna are two of them. (WP, "Rigvedic deities")
A hymn of the Rigveda further states "Devav asura" (Asuras who have become Devas), and contrasts it with "Asura adevah" (Asuras who are not Devas). In Hindu mythology, each Asura and Deva emerges from the same father (Prajapati), share the same residence (Loka), eat together the same food and drinks (Soma), and have innate potential, knowledge and special powers. Their choices and actions make the difference. (WP, "Asura")
Thus, the divisions between gods, devas, and antigods, (titans, asuras, or demons) were not clear-cut in the olden days, and results linger on despite much demonisation of some beings we hear of.
When gods and antigods cooperated to churn the Ocean of Milk (the Milky Way) on the back of Visnu in the form of a tortoise, at first poison came out as they were churning. Then came a goddess of wine. Then came a lovely tree, a jewel, a cow, a horse and an elephant. They were followed by beautiful, dancing women who became dancers of heaven. The goddess Lakshmi or Shri came out next. At last came the originator of Ayurveda, or medicine, out of the churning. He had a pot of amrita (life-giving drink) in his hands. Half of the amrita was given to the gods and half to the antigods.
But Vishnu did not want them to have it, and quickly became a beautiful woman, so enchanting that the charmed demons said, "Pretty lady," they said, "Take the amrita and serve it to us. Marry us."
Vishnu took the amrita, but served it to the gods instead of the demons. Only one asura, Rahu got some of it. He turned himself into the shape of the moon-god and got some of the amrita to drink. However, the Sun and Moon saw what happened, and told Vishnu of it. As a result, Visnu chopped off the head of Rahu with a sword. But Rahu had got amrita, and could not die. [This part of the myth is anchored in Vedic astrology about Rahu and Ketu. They are the Moon's Nodes in constellations, and so on, and a cycle around the Zodiak takes about 18 1/2 years.]
Anyway, the gods became more powerful than the demons, won over the demons and got heaven back from them.