No its for the sexy sound!
YouTube - Robispec Nissan 370Z single exhaust
the twin exhausts I have seen are 40-50 mm the single is 3" = 77mm
the 3" flows 4 times what a 40mm single 3 times a 50mm and 2 times a 60mm would so 40mm + 40mm = 2x the flow...vs 4x with the "BIG single"