Design wedge heads
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For starters, the soles are different. People do more with wedges than they do their 6-irons.
The Ping G20s I bought when I first took golf back up had a full set of wedges (PW, GW, SW, LW). It wasn't long before I swapped out the lob wedge for one that was the same loft and nearly the same bounce, but with a different sole grind that was a bit more accommodating for different kinds of shots. I really only used the PW, GW and SW for full swing shots like a 10, 11 and 12 iron. But I wasn't getting as much flexibility as possible using the clubs that way. While most people still make full swing shots sometimes with their various wedges, the real utility of a wedge is in partial swing finesse shots, and as noted above you really don't need the big cavity to help with forgiveness in most of these kind of shots.
Besides, it gives us golfers something else to tinker with.