In this case, they’re being used to create a basic PWM motor controller. The trick is to set the 555 up in astable mode, and use diodes and a potentiometer in the charge/discharge loop.

Once his son showed him an Arduino ... calls knob steering, a potentiometer drives the existing hydraulic pump, which he controls with a Polulu Qik serial DC motor controller.

This DC servo motor controller can also be used with an external potentiometer or the integrated on board potentiometer. For applications with existing PWM signals available this can also be used ...