For the same reason some on goes out and spends huge chunks of dough on music, or pc stuff. It is a past time, a game to play to entertain yourself and your opponent. Then there are those who do it profesionally, one can make a rather large amount of cash by selling the cards to collectors and players. Players can pull in a large amount of cash by playing on a pro tournament, the purse for the MTG world title was 200k USD, last time i looked it up. There really is no point to it other than a stimulating round of stratigic game play, or , in the case of pure collectors, a chance to meet and talk with people who normaly wouldn't cross your path.
Even further down the path, we TCG players do ALOT more when we meet then just play these games. We hang out together and do every thing you yourself probably do with your friends. But, in those instances when you have to wait for something, rather than sit there bored or breaking something because we are bored, we bust out our decks and have a quick game to pass the time.
Raust-00gryphon