What is it that attracts the eyes of those who behold a beautiful object, and calls them lures them towards it, and fills them with joy at the sight? ... Almost everyone declares that the symmetry of parts towards one another and towards the whole, with, besides, a certain charm of colour, constitutes the beauty recognized by the eye, that in visible things, as indeed in all else, universally, the beautiful thing is essentially symmetrical, patterned. –Plotinus, Enneads, I, 6 1, Translation by S. MacKenna, with slight changes by Gisela Richter from her Handbook of Greek Art)
Plotinus, a 3rd Century CE philosopher who wrote in Greek and lived in Alexandria, is considered the founder of Neoplatonism, which was an important and influential approach to the Classical philosophy of Plato and others, eventually influencing both Christianity and the Renaissance, particularly Renaissance artists. We need not dwell on the isms here, but we should recall the idea that we are drawn to beauty by its patterns and symmetry, and in particular by the way parts relate to parts and to the whole. Even if we do not subscribe to this notion of beauty, and there is no particular reason why we should, the idea can help us to see, firstly, that works of art are composed of parts, that the parts relate in some fashion to each other, and that they relate also to the whole - even if the whole, in our experience of it, is fragmentary - and that something about the patterning and arranging of parts will involve how some elements or features are parallel with others, often in symmetry or balance (or, conversely, asymmetry and imbalance). Parts relate to other parts when they are juxtaposed, when they are similar for some reason, and when they contrast with each other.
It has to do, then, with the nature of the relationships that make up the composition, and also, significantly, with the nature of the relationships (in figurative art) between the work and the model represented - art and reality -and, of course, with the relationships we, ourselves, create with the work itself - art and us. We are not just looking, then, but exploring, and our exploration takes place both across the physical object and through the abstract nature of the its composition.
And so, or, hence, to the question of art generally. What is it? No answer here. Where does it come from? From us. Why? I expect that, like many things, there is an answer to be found in our evolutionary background - not that I can identify anything further than that. There is something about talent, skill, and ability as such that we admire and wish to preserve, to encourage, in ourselves and in others. As Oscar Wilde said, The only excuse for creating something useless is that one admires it intensely.Whether the talent is for mimicry, making food, running fast, telling stories, or anything else matters less, I think, than the fact of the talent itself, and how it makes people feel to indulge it, both as creators and as consumers or audiences. Also, a talent for creating something that makes another sort of world, a virtual reality, is likely to be adaptive because of its useful and pleasurable qualities.
How does this tie in with the patterns and symmetries of Plotinus? Our own bodies are patterned and (more or less) symmetrical. Works of art that skilfully recreate a sense of pattern and symmetry, whether it is based on our bodies, on our perception of our bodies, or on our perception of the world in which our bodies function, is likely to make more sense if it derives from a patterned and symmetrical sense which we can, in Wilde's words, admire intensely. The work, whatever it is, becomes patterned by our response to it, as we make it and as we engage it, and a sense of symmetry derives, if from no other place, than from the relationship between it and us, back and forth.