yeah, they help with controlling the position.
Once the robot has recorded the map into its memory, it might for instance know that it has to move forwards 45cm and then turn right. Using stepper motors and knowing how many steps per rotation along with the circumference of the wheels will let it be able to move 45cm whereas if you use modified servos you can only turn them on.
You could use encoders along with motors or modified servos but that means the microcontroller has to deal with a lot of interrupts, most mouses progressively use stepper motors. (you cant get exactly 45cm with stepper motors because they all have a max number of steps. say you have a stepper motor that steps 56 times per rotation and 45 cm falls inbetween steps 51 and 52 and not dead on either of them, you lose a bit of accuracy).
You have to decide what level of accuracy you want before choosing the stepper. The more steps the more accuracy or resolution you get from them but it takes longer to complete a rotation. its a play off between speed and accuracy .....again ..... it always is