*Lower front end = better aero position
Curvy seat tube = gets the rear wheel as far forwards as possible (same as seat tube cut outs) for better handling, because the seats tend to be very far forwards.
This is what they were designed for, however they now are used only by hipster types (you can't race them any more afaik), so they are no longer "benefitting" the rider, they are ridden to be different/cool/look good.
*how do they benefit the rider?
*Lower front end = better aero position
Curvy seat tube = gets the rear wheel as far forwards as possible (same as seat tube cut outs) for better handling, because the seats tend to be very far forwards.
This is what they were designed for, however they now are used only by hipster types (you can't race them any more afaik), so they are no longer "benefitting" the rider, they are ridden to be different/cool/look good.