During my rides, I have my "trip computer" page set up so I can track my distance, speed, average speed, max speed, time of day, trip time, and moving time (there can be a BIG difference in how long you're riding vs. actually moving!)
You can also...
Track a map of your ride (useful for cross-checking with your ride map in case you get lost)
Use the same map to see major roads and geography (you can purchase a more deluxe map that can be used like a driving GPS)
Mark waypoints and see your distance and direction to them (coordinates that are useful for things like geocaching and rock hounding when you have to find a certain location in the middle of nowhere)
Makr locations of interest
Upload your maps
There are a lot of things I haven't tried, but I LOVE my GPS. And the battery lasts forever Well, at least a month or so, and I don't have to worry about it running out like a phone. I like to keep my phone battery as high as possible in the case of an emergency.