The rotary encoders are important for measuring distance travelled and the relative speed of the two motors (they won't be exactly the same). To make the car go in a straight line, you need to use encoders to continuously monitor the motor speeds and adjust them to be equal.
Wait, is this required even when I'm manually controlling the robot? (movement commands set by code, lets say that when I press the forward button, it goes at 200 in analog) do the motors not go exactly the speed you set them to go, so you have to implement a controller?
I'll also look into the line tracking.