I think the problem is that, now, POST means something more like "send the resource a message, as if it were an OOP-style object." Anything can happen when you POST.

PATCH is just, well, a patch—;)—to get back to the original semantics of POST, since with people using POST the way they currently do, those semantics (for things like caching) are unreliable.

POST is the wildcard verb when you can't use anything else. PATCHing seemed to be a common enough use case to warrant standardising. PATCH, combined with ETags, offers idempotentcy meaning that if you are unsure of success you can simply repeat the operation. This can really come in handy in a distributed system working over a network. (PUT and DELETE have this property, POST does not) Also, when intermediaries can understand what you're doing, they can handle caching much better, invalidating what they must, etc. Again, POST does not give you that luxury.