I can't imagine riding without SOMETHING telling me how far and how fast I went. Granted, you could figure out your mileage with google maps, and keep time with a wristwatch, but like others have said a simple cyclocomputer (i.e. cateye wired models) can be had for $30 or so. For that little money (less than a tire or a couple weeks worth of clif bars), I don't know why you'd NOT buy one.