So I've been studying bike geometry for a while now and have stumbled upon something that seems to be done only because that's how it's always been done. Set-back seatposts. If you go back 40 years and strike a straight line from center of BB to center of saddle clamp, you'll find that the line travels through the rear wheel. Since steel is fairly difficult to manipulate into complex shapes, my guess is that this is where set-back seatposts were born. It allowed for the rider to sit back enough from the BB to generate power.
Now take today's composite materials that can be molded into just about any shape and I have to wonder why set-back seatposts even exist. The "seat-tube" can effectively wrap around the rear wheel and then travel up to the saddle clamp. If the saddle clamp is perched directly above the center-line of the seat-tube, it should be exactly 73 deg. (or whatever the geometry states on your particular frame.) But if you use a set-back seatpost, you're effectively changing the angle from 73 to something closer to 71 or 70.
Maybe I'm missing something here.