It does add something though. It informs new users that it is both possible to delete a bad post, if they feel it is not salvageable, and it teaches them that when they can't salvage a post it's actually a good thing to delete it.
We of course don't see the benefits of this very often. They're invisible; that's the whole point. There is a lot of garbage that we aren't seeing because people learned to take it out themselves.
Yes, this does get abused in which people create garbage just so that they can get credit for picking it up. Fortunately, this doesn't happen all that often, when it does happen, at least they picked up the garbage, and finally there is only ever an incentive for this (mostly benign) abuse once per account. If users were going around creating lots and lots of trash over and over, just to later delete it, it would become disruptive. As it is, the very rare abuses simply aren't that significant or disruptive.