For racing in the dirt you should make two holes in the tread of each of your tires so that any dirt or water that has entered the tire and rim can spin out by the holes you made in the tread.

Also those small holes on the side of the rim are for glueing the tire to the rim for added strenght on the side wall so that you don't loose a tire bead as easily as the other rims do.

Here's a pic of Mayfiels SC10 4x4 with the holes in the tire tread: