Comment by estebank

To give you an example, rustc has an AST, and a high-level intermediate representation tree called the HIR. The former can encode anything that is syntactically valid (and some things that aren't), while the HIR only encodes things that are valid and transforms things like `if foo {} else {}` to `match foo { true => {} false => {}}`. That makes this internal representation more fundamental, there are fewer constructs to care about, and allows unifying some validations that would otherwise have to be written for all the alternative user visible constructs. It also makes later stages of the compiler not have to care about how code was written, just about this higher level representation.