The fillers, as noted, come (or at least they used to) with the 26" and 28" barrelled guns, while the "Sporting Clays" model with 30" barrels did not have them.
The fillers are easily removable, and are intended for hunting use in the brush, keeping twigs, grass, and other stuff from getting stuck between the barrels. How necessary are they??? I dunno.
If the gun is to be fired a lot in a short period of time, as in trap, skeet, 5-stand, etc. having no fillers is supposed to allow the barrels to cool more quickly.