What's the purpose of writing about other works?

I understand writing to make a point, I understand using another written, or whatever medium, of piece from someone else as support.

But I don’t understand interperative essays on books and poems, and having assignment after assignment of writing about dictionsyntaximagery.

I guess it would be one thing if I was able to just pump these assignments out, but I don’t seem to be able to, and the more I try to and the more I get stuck the more I wonder where the necessity is in it. The piece being disected is better and deeper and says more than any essay taking out a single thread from its essence ever will… So why do we do it?