TC,
The keep most of the mud out. Soem of the discs and wheels don't have the exact tight fit, but for the most part they do that job fine. Just be careful when washing the tires/wheels with them in. I've had some dust and dirt get behind the discs and throw the balance off.
As for the tracks,
There's a lot of different tracks that I have been to where I can't really tell if they work better with or without them in. I do know that on a high bite track, the corner speed improves with them in. I think of them to be ment more as a tuning aid, but I'm lazy and once I put them in, I'll leave them in. LOL.