Better traction and saves bearings...
Ballencing your wheels makes for better traction. You will see realy bad unballenced wheels bounce up and down up the straights and put heaps more stress on the bearings. Ballence them and you will notice the difference for sure. A good ballenced wheel when done right will spin longer, then stop in random spots everytime. 90% of all wheels I have got new have had to be ballenced. I use modelling clay or plastasine, they both work fine and are cheap and easy to get. Once you do it once you will be hooked.