Every Alden New England shoe carries a tempered steel shank, precisely contoured and triple ribbed for extra strength. Truly the backbone of a fine welt shoe, the shank provides the proper support and shape so necessary when your day involves much walking and time on your feet.
In other words, the shank makes the shoe more comfortable and provides support for the foot. Contrast this to what Allen-Edmonds writes about their construction, which is shankless:
Unlike most shoes, Allen-Edmonds require no rigid shank for support. Shankless, our shoes move naturally with your feet and feel great the first time you wear them.
They don't put shanks in their shoes, you see, to keep them flexible and comfortable.
Both pieces of marketing are so much hokum. Regardless of the method of construction, shoes shouldn't flex in the area of the waist. Any shoe that does flex in this area while the wearer is walking will quickly be torn apart. The shank helps to provide this rigidity, and it ties the heel of the shoe to parts forward -- there is nothing else to do it if the heel is close-cut or the shoe is Blake- or Blake/Rapid-constructed. Shanks do indeed provide support, but for the shoe, not for the foot wearing the shoe. Allen-Edmonds shoes don't need the rigidity and the stability that shanks provide because they all have 360 degree welts. This welting both ties the heel to the rest of the shoe and supports the shoe's waist. One method is not inherently superior to the other except in the realm of aesthetics: a 360 degree welt is inherently clunky-looking. It's impossible to make a sleek heel or waist on a shoe that has a 360 degree welt. This limitation doesn't necessarily make any difference on, say, a gunboat of a shoe like a long wing; but it certainly does make a difference on something that is supposed to be more refined.