It's useful if you have some loop running and you want to stop it for some reason in an orderly fashion. Say you were processing several years of data but now w

It's useful if you have some loop running and you want to stop it for some reason in an orderly fashion. Say you were processing several years of data but now want to stop when the first year is completed for some reason. Maybe it's taking too long and you'll rather keep working with partial results or whatever. You can easily recompile the processing function to raise an error when it reaches the stopping point. Or maybe you noticed that the loop will eventually hit an edge case which is not properly handled currently in the code. You can fix it while the loop gets there. And of course for GUI programming. You can redefine interface behaviours on the fly.