In short, an encoder gives you much better control over your speed. The advantage of using an encoder is that it 'closes the loop' with your throttle, so your car can reliably command an actual velocity rather than just issuing a motor control which will produce a faster or slower velocity depending on the slope of the track, the surface or mechanical friction in your drive train while turning. This encoder can be on the motor, on the main drive shaft or on individual wheels. Odometry is a way to calculate the speed and distance travelled of the car by measuring the rotation of its wheels using a sensor called an rotary encoder.
