Well, "need" is a strong word. The features are definitely helpful though. With large programs, there can be a really long distance between where something is

Well, "need" is a strong word. The features are definitely helpful though. With large programs, there can be a really long distance between where something is created/modified and where it's used. If there's a bug you catch that occurs where the object is used, reversible debuggers help you trace back to where it's created, or who modified it last. If you're at a place that's hard to put a useful breakpoint (e.g. a widely used function), and need to answer a what-if question, being able to manipulate state from the debugger is super useful. You might not be able to easily insert code to the program to create that state without messing up everything else (again, example is a widely used function, where inserting code could mess up other users of the function).