I am trying to learn Geometric invariant theory like it was introduced by Mumford. But I do not have a strong motivation and so I want to know the reason of studying Geometric invariant theory. I just know Geometric invariant theory plays an important role in the construction of moduli spaces.

What's a reason for studying Geometric invariant theory? And, what are related (big or small) problems with Geometric invariant theory in algebraic geometry?

Any answer will be very helpful for me.