They are sometimes used as a function generator. For instance, if you used a particular geometry, you can input a constant speed, and the output it s sine wave.
They basically act as cams, but instead of having a variable, linear motion output, they have a variable, rotary motion output, So they could be used for a lot of things such as rotary kickers that speed up when the flipper is near the ground or to move an arm (the closer to horizontal it is the harder it is to turn, thus, a variable gear ratio that has a higher reduction when the arm is horizontal than when it is vertical is ideal)