Debugging for one. Same reason you might want to attach to an already running C/C++ process with GDB. (This is perhaps less less powerful for that use (since

Debugging for one. Same reason you might want to attach to an already running C/C++ process with GDB. (This is perhaps less less powerful for that use (since it would currently lack any sort of manual flow control), but I think the benefits of it actually being a REPL might offset that for some uses.)