They are the tubes that join the rear dropouts with the seat tube, transmitting the shocks received by the drive wheel to the entire frame. Among the most important tubes for the bicycle comfort and even the thinnest.
The use of perfectly straight tubes has been abandoned to favor damping capacity.
Fiber seat stay is almost completely adopted the monostay logic: made in a single structure and assembled at the brake jumper. It is useful for strengthening the structure.