Guimard likely was moving the saddle back to close the hip angle which activates the hamstrings and the glutes. Remember at that time, bikes were only build a certain way and had relatively high bars compared to modern road bikes. They didn't have the option of dropping the bars to close the hip angle. Open hip angles (high bar + forward saddle, or knee in front of pedal spindle), stress mostly the quads. The more closed your hip angle, the more power you can put into your pedals because you activate three muscle groups instead of one. On the other hand, it is harder to breath and so it somewhat limits your aerobic output.

The trick is to balance these out. Road sprinters and flat lander rolleur types are relatively neutral to closed hip angle. Climbers have an open angle they can then close by sitting on the back of the saddle if they need the power (they get away with this because their torque to the pedals is relatively low during a climb; power riders have to make their upper body super rigid to drive power into the pedals and so have no choice but to sit on the front of the saddle - hence the term "on the rivet"). Where position your body over your bike depends a lot on your strengths and goals for your riding.