By now, you may have seen differences in the outputs, that is, a veer. In only 400 mm, my left side is around 20 mm behind the right, an error that is climbing. Depending on your motors, your robot may have some veer too. It is rare for a robot to have driven perfectly straight. We'll use the sensors to correct this.
Note that this is still dead reckoning; slipping on surfaces or incorrect measurements can still set this off course. This works better on wooden flooring or MDF boards, and poorly on carpet.