More often than not, it has to do with the wheels: a new tube because you rode over something sharp, new tires after you've worn down the old ones with many miles of rides, or even a new rim if yours got bent from a minor accident. Brake and drive chain components are also fairly common purchases. And, many people who don't like the original seat (or saddle, as the experts would refer to it) opt to replace it with a more padded, ergonomic design.