I'd suggest altering your question. Everybody has a main. If you don't have a main, you don't play Runescape. Either that, or you don't know what a main is. To answer the question, people have different accounts for a change of scenery, or to relive fun parts of the game. I have a quest pure, because I love quests, but already have a quest cape on my main. People have pures because they're generally better at Pvp, and they think pking is fun.