What are the advantages of having the ribbing around the middle of the can? Is it for expansion so that the can acts like an accordian? Does it matter what type of food is in the can as to whether it should be ribbed?
The ribbing adds strength. This allows the manufacturer to use thinner metal to make the can. By the same principal, construction workers use I-beams in buildings rather than solid steel bars the same height and width as the I-beams.
The same principle applies to corrugated cardboard.