I was a few years into amateur self-teaching at that time. It helped me think through problems recursively, which was a lot of fun, but my biggest takeaway was learning to think of problems by representing the data with types, then stubbing out functions that dealt with the different types that I needed to solve the problem. Although it's done with an untyped language, it makes you think through the type signatures of all the functions you write. I discovered that whenever something didn't work, the problem was almost always that I misunderstood the data or the type. I also learned how to deal with problems with recursive algorithms through trees and nested data structures like s-expressions. That opened up a whole world of parsing and evaluating.