Optimum Versus Body Fortress Why Is It Better/more Effective To Place Heat Registers On The Floor Versus The Ceiling?

Why is it better/more effective to place heat registers on the floor versus the ceiling? - optimum versus body fortress

Is it because the air is warmer to the cooler floor to ceiling, of course? But the hot air, collapsed the roof at all? The question is whether the ground is the best option, but I do not understand why this is so.