There's little direct benefit, since the internal structs pretty closely model SQL. But having the language compiler a part of the Postgres process does help. It gives you easy access to the database's structure, so you know the type of every identifier, what columns tables have, what functions are available, etc. You can then do your own (better) error reporting and, more importantly, move away from SQL's semantics.

For example, I want to have universal broadcasting of operators on subquery results, array values, and aggregated columns. To do this, I need to know which of these the operand expressions represent, which is slow or impossible with transpilation.