Why? I just mount my Garmin on the front of the bike, which gives me anything I could want other than cadence (which I occasionally check the old-fashioned way). But maybe you don't use a gps watch?
Actually, I was riding with a GPS watch and this cyclometer doesn't have cadence, it's about the cheapest cyclometer made. What's it for? Not really sure. It shows speed and distance in a place that's easy to see, so that's something. I'm pretty sure I bought it because I was trying to get my order up high enough to qualify for free shipping. And it means I don't have to use the GPS watch if I don't want to.