Not actually cleaning, just rinsing.

The logic is to prevent:

  - Chunks of fibrous vegetables (e.g.) from clogging the dishwasher filter
  - Wet sauces (or egg) from drying and hardening over the hours/days before the next dishwasher cycle, and becoming more difficult to remove

And many of us grew up with older dishwashers that didn't work as well as newer ones.

All of that said, modern dishwashers actually monitor the water (clarity, turbidity?) to determine whether the dishes are sufficiently clean. If you rinse your dishes too well, the dishwasher will prematurely think it has accomplished its goal, and reduce time/temperature to end the cycle early. This is why manufacturers recommend against rinsing or pre-cleaning.

In my household, we have a pair of zealous canine precleaners, who do an excellent first-pass job. The dishwasher's only responsibility is to rinse and sterilize. :)

You will after you have to pull the dishwasher out, turn it upside down and partially disassemble it to clean the filter which is blocking the flow of water intended to rinse your dishes.

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.

I do it and recognize that the logic is flawed, but it's a habit and just looks and feels correct at this point.