Each Purana is in the main devoted to a particular devata. In the Siva Purana it is stated: "Siva is the Supreme Being. He is the highest authority for creation, sustenance and dissolution. It is at his behest, and under him, that Visnu funtions as protector. Visnu is a mere bhogin, trapped in Maya. Siva is a yogin and jnana incarnate. Visnu is subject to Siva and worships him. Once when he opposed Siva he suffered humiliation at his hands". Stories are told to illustrate such assertions.
In the Vaisnava Puranas you see the reverse. They contain stories to support the view that Visnu is superior to Siva. "Is Siva a god, he who dwells in the burning grounds with spirits and goblins for company? " these Puranas ask.
In each Purana thus a particular deity is exalted over others. It may be Subrahmanya, Ganapati or Surya. Each such deity is declared to be the Supreme God and all others are said to worship him. When, out of pride, they refuse to worship him they are humbled.
Doubts arise in our minds about such contradictory accounts. "Which of these stories is true? " we are inclined to ask. "And which is false? They cannot all of them be true. If Siva worships Visnu, how does it stand to reason that Visnu should adore Siva? If Amba is superior to the Trimurti (Brahma, Visnu and Mahesvara), how is it right to say that she remains submissive to Parameswara as his devoted consort? The Puranas cannot all of them be true. Or are they all lies? "
Logical thinking seems to point to the conclusion that all Puranic stories cannot be true. But, as a matter of fact, they are. A deity that suffers defeat at one time at the hands of another emerges triumphant on another occasion. And a god who worships another deity is himself the object of worship at other times. How is this so and why?
The Paramatman is one and only one. He it is that creates, sustains and destroys. And it is he who exfoliates as the the many different deities. Why does he do so? He has not cast people in the same mould. He has created them all differently, with different attitudes, the purpose being to make the affairs of the world interesting by imparting variety to them. The Paramatman himself assumes different forms to suit the temperament of different people so that each worship him in the form he likes and obtain happiness. This is the reason why the one and only Paramatman manifests himself as so many different deities.
Everybody must have firm faith in, and devotion for, his chosen deity. He must learn to believe that this deity of his is the Paramatman, that there is no power higher. That is the reason why each manifestation or form of the Supreme Godhead reveals itself to be higher than other forms or manifestations. It is thus that these other forms are shown to have worshipped it or suffered defeat at its hands. Altogether it means that each deity worships other deities and is in turn worshipped by others. Also each god suffers defeat at the hands of other gods and, at the same time, inflicts defeat on them.
In the Saiva Puranas all those aspects that proclaim the glory of Siva are brought together. Similarly, in the case of the Vaisnava Puranas that deal with Visnu. Amba, Subrahmanya and other deities are each of them dealt with in such a way as to show him or her to be the highest among the devatas.
The purpose of exalting a particular deity over the another is not to depreciate the latter. The underlying idea is that a person who worships his chosen god has unflinching faith in him and becomes totally devoted to him. Such exclusive devotion is called "ananyabhakti". The idea here, however, is not to regard other devatas as inferior to one's own chosen deity- an example of "nahi ninda nyaya".
Those who are capable of looking upon all deities as the manifestations of the one and only Paramatman have no cause for exclusive devotion to any one of them. It is only when we think that one deity is separate from- or alien to- another that the question arises of giving up one for another. If we realise that all are the different disguises of the One Reality, the various gods and goddesses potrayed in the Puranas, with all the differences among them, will be understood to be nothing but the lila or sport of Supreme Being. It is the One alone that seems divided into manifold entities. This is to help men of various attitudes and temperaments. If this truth is recognised we shall be able to see the stories in the Puranas- stories that seem contradictory- in the true light.
In the story of Banasura we see that Siva is vanquished by Krsna. But in the story of Tiruvannamalai, Visnu meets with failure in finding the feet of Siva. Both stories must be treated as truthful. The first is to make devotees of Krsna worship him as the Paramatman and the second to make devotees of Siva adore him similarly. Although we think that one is winner and the other the loser or that the one is superior to the other or inferior to him, the two know themselves to be one. Does one triumph over oneself- or does one inflict defeat upon oneself? So all this is play. The Parmatman indulges in sport assuming multifarious forms.
The purpose of the Puranas is to show people the right path. Pativratya is a virtue that is of the utmost importance. Amba herself exemplifies it. The Parasakti, the Supreme Power that she is, remains subject to her husband. Faith and devotion must grow in the world and for it the Lord himself must show the way. This is why in some temples Visnu is represented as a worshipper of Siva and in some other shrines Siva is seen as a devotee of Visnu. The same with other deities. I have spoken more about Siva and Visnu since Saivism and Vaisnavism are the two major divisions.
To sum up, if a deity is glorified in the Puranas, and stories told in support of it, it is to create exclusive devotion to him as the Paramatman. And, if any god is potrayed as inferior to another, the true purpose of it is not to denigrate him but to develop unflinching faith in the latter.
For a general background, please see here