A refatoring tool might want to offer semantic renaming. If 'x' occurs in many places textually in the source, the only way to be sure that a rename operation only acts on the same variable, (rather than just the same string) is to (at least partially) parse the source, act on the resulting AST, and use source information in the AST to apply the change to the source.

So, to avoid maintaining a separate parser, it would be nice to grab an AST from the compiler - but that AST has to match 1-1 with the source to be useful for refactoring.