When seatposts came in one style, you could use seat-tube angle to provide more or less saddle layback over the BB. Now that we have a wide variety of seatpost laybacks, from inline to extra-long, you dont need a special seat-tube angle to position your saddle relative to the pedals.
I switched from a std model to an in-line model to emulate a steep seatpost and reduce my horizontal distance to the pedals.