I think people don't want to clog their dishwasher with pieces of food. If I have a couple pieces of spaghetti, a part of a leaf and half a chickpea stuck on the plate, I would remove them with a paper towel. Not sure why anyone would rinse it afterwards, though.

That reminds me of people who clean before the maid comes. I've never had a maid, but I've read that people do the easiest things themselves so the maid, who is paid by the hour, has to do the harder things only.