MX Tire/Wheel Explanation

This may be a dumb question to ask, but this has been something that I've always wondered. Typically you see MX quads with 19x6x10 or 20x6x10 front tires and 18x10x8, 18x10x9, 19x10x8, or 19x10x9 rear tires. There are others, but I think those are the most popular sizes, correct? What is the benefit of using the different size tires, rims, etc? It also appears that beadlocks are a must? At what point is it necessary to have beadlocks? At which point is it necessary to have them on the front and the back? I would like to start getting into MX and I'd like to see what size of tire and wheel people are using and why. Also, what brand/style of tires and wheels do you recommend? I'm currently running 21x7x10 Kenda Front Max's and 20x11x9 Kenda Gnarly's on ITP Baja wheels. This probably isn't the best setup to use, huh?