To know how many miles are on your bike? Aside from just wanting to know, it can help plan and budget for tire and chain replacements. Having a clock on the handlebars is really nice this time of year, when my watch is buried under gloves and sleeves, and I don't want to pull them back and expose my skin to the cold. Saturday, I spent much of the day on the bike; it's kind of nice to know that I rode 70 miles, instead of just "a lot." In theory, it might even help a person avoid a speeding ticket going down hill - I've at least read about cyclists getting those.

Plus, it's not like you have to wind them up like old watches, or actually do anything. You spend $30, once, and then a bunch of information is available if you ever want it, but pretty much out of the way when you don't. I'm not sure why anybody would want not to have one.