Local simulations, testing, etc. A REPL is much easier to connect than gdbstub when the link has 20 minute RTT.
Remote Agent was first for a) being able to be interactively debugged at remote b) actually exercising that, also c) IIRC it was already designed as basis to provide self-healing kind of ability to the system (or I might be confusing it with another lisp system from Ron Garret while he was at JPL)
But it would be hard to identify any long-lived space probe whose software was not updated after launch. There are famous stories about Mars rovers.