Because if you need to brake and it clamps down on dirt/grime on top of the rim, you don't want that being rubbed against the rim as it slides under the brake.

The groves allow the debris and stuff a place to go. It isn't "much", but you're saving the extra 3-5cm of scraping that the dirt would otherwise do.

I would also say it has something to do with the bending of the rubber when you apply the brake. The slots allow the rubber to bend a bit before fully catching and applying all the stopping power. I don't know how much, or if, that matters, but you could easily test it out and see. It would probably allow for more modulation if you toe-in your brakes.