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.
To me, the most interesting project to build would be a line-follower robot. Some ir emitters & sensors to sense a white or black painted line or tape on the floor. Once you have the car following the track reliably, including sharp turns and intersections, you can lay out a maze, and have the car find its way through the maze without previously knowing the maze layout. Have you seen the "micromouse contest"? Search for it on YouTube. Amazing!