It’s hard to get a good feel for who your clients are before you take them on, so sometimes you end up with clients that require a bit more hand-holding and a few more revisions than is ideal.
As such, your fees need to reflect the possibility that each project might take longer than expected because of this unknown. So when you have an awesome client and you don’t have to spend that extra time on the project (and in fact, can finish even quicker than normal), it’s nice to be able to give that money back in the form of an “awesome client discount”.
Your client was already happy to pay you your normal rate, so the fact that you were able to give them a little gift back for being awesome may just make their day, and a lifelong client out of them.
Besides that, it’s just nice to be able to be generous, spread joy for no reason other than philanthropy itself and put a smile on someone’s face.