The flutes act in the same principle as the corrugation of cardboard in a cardboard box. The flutes add strength. One other point is that if the seat tube wall thickness is uniform throughout it's circumference, the flutes also add weight. Why? Because the total linear measurement of the tube circumference is greater when it's fluted. It could also be that the seat tube maker was using a "thinner" than usual alloy and employed the flutes for strength. In this case many would say that the flutes reduced weight. Not true though, it is the thinner alloy that reduced some weight, but the flutes cancel some of that weight savings. This is similar to tubes that are butted. A tube maker uses a thinner tubeset. That reduces weight. But to add strength to the thinner material they butt the ends. This increases the tube thickness, adding weight. But, many would say that butting reduces weight. Not so. The thinner tubes provided a weight reduction, and some of that weight savings is cancelled by the neccassary butting.