They are usually on the larger a/c, ie....773, 346 380's etc to aid the pilots in actually taxying the thing. Common to have a camera on/close to the nosewheel so the pilots can keep the taxyway centreline accurately and others have them on the tail looking forwards onto the fusalage, again as an aid to driving these long things around tight turns on the tarmac.
Great for passengers too i guess when its piped into the cabin on IFE