Huge size and weight advantage (10-15% shorter, and 20% lighter)
potentially 6-7inches shorter overall length of the firearm (1-1.5in stock plus another 5-6 in shorter barrel). Say what you want, but a firearm an extra half a foot shorter is simply much more manuverable in the woods, quicker to reposition on flanking birds, less likely to get hung up on brambles/ briers, etc.
Combine the shorter overall length with a firearm who's reciever is often build lighter/ smaller and there can be a significant weight reduction (usually 2lbs or more). Miles per day adds up to a couple hundred miles hiked per season, and over time that helps. Plus, the lighter weight makes it easier to position the firearm and even shoot one handed, leaving the other hand free to work a pot, scratch leaves, etc.
If one isn't sacrificing clean/ ethichal killing power by going with a smaller firearm, why would anyone choose to hunt with a heavier/ more cumbersome firearm is the real question?
The only downside... the shorter barrels don't throw quite as dense a pattern (often more fliers from the ultra short barrel) in my experience. TSS makes up for that (I get the same pattern from my 21in bbl 20g #9 TSS 1.5oz as my 26in 10g 2 or 2.25oz (I can't remember) #7 15g/cc heavyweights)