Sometimes it's a cover your rear tactic by shady sellers. If the buyer has an issue with something that was sent, he may thing twice about leaving a negative feedback, because the seller can respond with a negative as well- even though it wouldn't be warranted because the buyer did everything right.

In my case I've waited as a seller sometimes, because it also helps remind the buyer to leave a feedback. Too many times I've left feedback and never received one from a buyer. They forget, or don't care. By me waiting, it reminds them because you can be sure they want to get that positive feedback.