Why 33billion Gods in Hindu religion?

The number bandied about is actually 330 million or 33 crores.

Philosophically, most Hindus would (I think) say that they are all different forms of the same Absolute. Ramakrishna used to give the example of the same river from which Muslims, the English (i.e., Christians) and Hindus draw water. In his example, Muslims call the water "pani", Hindus call it "jal", and the English "water", but it's the same thing. Some systems might claim that their deity is supreme and the others are not though.

Warning: what follows is the original research of a casual dabbler.

From a psychological sense, it's obvious that each Deity has a different "feel" to him or her. Following the ideas in Tibetan Vajrayana, I think the purpose of the various forms of deities is tantric, by which I mean techniques to use your emotions to attain self-realization.

Take the deity of Allah - it has a very fierce feel to it. You cannot be a "friend" of Allah. You cannot make fun of him. If you must communicate with him, it can be done only through middle-men - the prophets. And even the prophet needs a middle-man - an angel! To Allah, we're all slaves. At least as per the mainstream Islam in most of the Arab world.

On the other extreme, take Krishna. There are supposedly different bhavas with which you can love him, and these have been elaborated in the tenth canto of the Srimad Bhagavatam. Every Hindu grows up with these stories. If Allah (the deity) is the fierce desert sun blazing in midday and would fry you if you so much as look at him, Krishna (the deity) is the same sun shining in the springtime morning in vrindavan.

So, while I am not aware of any meta-theories of deities within the framework of Hinduism, looking from outside, it seems that deities are designed to exploit our characteristics as biological and social beings in order to approach the ultimate goal: that of self-realization or God-realization. Using deities, by exploiting maya, we overcome it.

However. I actually think the rabbit hole goes much deeper than this. I suspect that those who truly understand the theory of deities is a very, very "spiritually" intuitive person, and as some Hindus might say, probably "self-realized". I am pretty sure that Shankaracharya was one such person.