Why do we need a motion coprocessor? Apple talks about the M7 and M8 motion coprocessors for the iPhone pedometer and other operations. 

My question is Why do we need a motion coprocessor? Surely this is just digital signal processing that the massively powerful primary processor can handle?
You have implied the reason in "the massively powerful primary processor". It takes electrical power to run the computer processing. Thus, if you need something to be on all the time, you want a smaller processor than the main one to save battery power.