If you live in a place without rain or snow, I guess you don't need 'em. In a lot of places, though (maybe moreso in the northeast? I saw a ton of it in New Jersey last year on vacation), you can get a kind of slushy hybrid monstrosity of precipitation on the roads that's messy, cold, and collects all kinds of dirt. I have been lucky enough to not ride in that yet, but now that I have fenders I'm glad to be prepared.