yeah, that reusability is a bit weird. functions can be used a lot as well.
as for the idea of recoding without other things changing: functions can also be recoded without the need of rewriting the rest of the system. they just get called, and return something.
I've been learning to write classes lately and I like the way you start to think in terms of grouping your functions, but I feel it might sometimes limit your thought pattern instead of extending it.