Arm length is needed to determine the location of the front of the bike with respect to your saddle position.  Then depending on the nature of your riding, your reach will also have bearing on the drop & design of the handlebar.

Your feet are levers on the end of your leg.  Although muscle type and body makeup can determine the crank arm length, saddle setback from the bb includes the entire leg hip to toe.  (Remember the stick figure?)