For me it is convenience (or laziness, take our pick). Turn it on, snap it in, push the button, and forget it. Get home, unsnap it, plug it in, look at everything in pretty colors and graphs, then clear it and put it back on the bike for next time.

I have one battery to worry about, the cadence pickup, and if it dies I don't really care because I stopped looking at that data a long time ago. No wires. No pickups (as long as you don't care about cadence like me). No surprises.