Wheel sensors
A wheel sensor lets the GPS computer know when I've stopped. Otherwise, the ride recording can show a random drift path around my stopping point. And it's a little more accurate on road speed, too. (My old Garmin can auto adjust the wheel size for the sensor, it's pretty accurate.) A wheel sensor is nice to have, but not required.

GPS accuracy:
Here's an example, with the outbound ride heading north, uphill. Then the return is downhill with a right hand turn. You can see which side of the road I was on.
Each dot is a recorded point at one-per-second intervals, closer together uphill, of course. 10 mph is about 14 feet per second.