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.)

I just did PyWeek (week-long python game competition), and this would have been great for seeing the effects of my code changes right away.

I still had a workable solution: I wrote a script that watched for any .py files that have been saved, and if so, ran the game for 15 seconds.

Well, if you have a Python-based closed-source application, and you want to debug that, extend or change its functionality, or find out what happens inside it can be useful...

Meliae at any point in the process would be awesome for long-running servers.