It can be used to monitor speed, distance, lap times, cadence, heart rate, power (if you have a meter), etc. Some can be used as to follow a route or to compete against a virtual riding partner. Phone apps can do most, if not all, of this, but they generally have poorer battery life and are not typically suited to exposure to foul weather. In my experience, accuracy on the phone was worse, too, but that was a couple phone generations ago. Whether or not it's worth it is up to you to decide.