I want to use them so I don't have to write very much code, so that I can easily reason about my program by directly substituting symbols with their definitions

I want to use them so I don't have to write very much code, so that I can easily reason about my program by directly substituting symbols with their definitions, so that I can use pattern matching against instances of that type, so that I can group them into type classes and know what operations I'll have available to work with them... etc.