You may not need to, but it can be worthwhile. You can get free flights and cash back on many cards. The normal reward rates are very low, but some cards have sign up bonuses that are worth a few hundred dollars if you spend a certain amount in the first few months. Many cards will keep giving you the sign up bonuses if you cancel and re-sign every couple of years. For me, the few hundred dollars is worth the trouble of churning the cards.