Knowing what the AST looks like without having to sit an think about it is huge. Maybe you're smarter than me and can instantly intuit it, but I think most people can't. I think that this is one of the reasons that macros are so widely adopted in LISPs but not in other languages that support them. Having an immediately evident AST makes it much easier to reason about, and hence to manipulate either through code or manually.
Edit: Improve clarity.