If your kemper is sitting in a rack somewhere in your studio where not everything is at hand, a software editor is really useful.
Even if not (as in my case) it can be useful. For him it might not be useful for whatever reasons. I have the toaster and I really would love to have an editor since I dont want to put the toaster up on my desk to dial in tones and tweak this or that. It just saves me space on my desk where I already have way to much stuff ;).
Another reason for me personally is the fact that you can speed up things with a well designed software. Espacially changing parameters would be way much faster if you could see als the parameters on one single screen. But all that is already mentioned in this thread :).