The longer the barrel the less visual lead is required in front of the intended target.
That's why almost all O/U seen on skeet fields these days are 30".
Doc
Can't agree here. The perception is the same for me and my guns. When I use a swing through technique (virtually all of my hunting), I find that the shorter barrels move faster to the target and as a result I need to see less lead with them on hard crossing targets than I do with longer barrels. The short barrels start moving faster (and stop moving faster too) than longer tubes.
The reason for popularity of longer barrels at the target games in general, not just skeet, is handling dynamics that are a combination of balance, moment of inertia, fit, etc. The longer barrels, in general and all else being equal, are easier to keep moving than shorter barrels. This makes it more difficult to commit a primary sin of wingshooting - stopping one's gun.