Needed for thick items
Even though the specs for the VP215 state that the maximum bag size is 10x13, that doesn't take into account the length of bag that is lost due to item thickness. While the 10x13 bags are adequate 95% of the time, if you are sealing something thick, like a boneless rib roast for sous vide, you have to include twice the item thickness lost in bag length when sealing. So, an 8-inch long roast that's 4 inches thick is going to require 16 inches of bag length to fit the roast, plus you need some left over to reach the sealing bar. You can't do that with a shorter bag. Plus, it can be trimmed if, say, only a 15-inch bag is needed.