This has always struck me as significant effort invested in making a bad user experience only slight less bad: the REPL overloads the meaning of "enter" to mean both "insert a new line" and "execute command", and then attempts to guess what the user meant by tweaking the grammar of the language. Why not distinguish between "insert a new line" and "execute command" as two distinct inputs, and eliminate all ambiguity ?

I haven't used it for that, but I have used it to absorb errors and continue. The error just gets placed into the AST and then a later phase can report on (potentially) many errors, rather than just stopping at the first. YMMV