For any robotics project it is nice to add a camera. Sure sensors handle the main controlling but sometimes it’s nice to have the camera option to see what your robot is seeing. The signal can be fed back into software like SimpleCV or OpenCV to allow the pi to detect faces, colours or a particular object.
The first thing to decide is which camera to go for. With the Pi you can use USB webcams or the purpose made cameras that connect directly to the CSI Port on your Pi.