Why is it important to study the great economists of the past?

Answer 1

It's not. Unless you need to to pass a class, in which case it is most important. It's far more important to study modern economics. While looking at what Aristotle or Malthus thought about the economy can be interesting and enlightening, their theories are largely discredited; it will not do much good in helping you understand the world.

It's like trying to get a good model of the universe by studying Newton. He was revolutionary, but you will be better off learning modern physics, which happens to be largely based on Newton's work. Economics has had too many cranks and discredited theorists to waste your time studying its past. Unless that's what you want to do, of course.