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

A lot of people don't know that dishwashers have filters that need to be cleaned regularly!

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. :)