I would assume that Draper, amoung many others who understand BB'ing, used a block under his heels for more quad development and to reduce the involvement of the arse/hips out of the exercise. I wouldn't believe that one of the main reasons would be for balance. Flatfooted squats will tend to build the glutes more and BB'ers of his era, and present, do not want a large wide arse. Flat footed squats will increase the flexability/strength of the ankles and calves, to a lesser degree, which most athletes favor for their sports.
As I suggested before, if you have a potentional for a weaken knee structure than squating on a block may not be for you. Only time and a increased workload will tell. If your knees are normal and do become stronger the more you squat, than extra heavy squating should not be a problem. So a general statement that block squating is easier on the knees is not always correct. Actually it can be very misleading. Good Luck.