AFAIK, bentonite is not a clearing agent like isinglass, chitosan, kiselsol, etc. In kits, at least, it is used to get rid of protein haze. As such, the kit companies have learned that it can be used at the beginning. This means that less bentonite is required, and less stirring of the bentonite is required. Also, the bentonite aids in fermentation by providing nucleation sites for the yeast.
FYI, when I first started making kits, some brands had large bags of bentonite that were added prior to the other clearing agents. (Boy were some of them fun to re-hydrate.)They also recommended vigourous stirring of the bentonite at that time, which additionally accomplished the degassing. One brand recommended racking from carboy to primary, then adding the bentonite, and vigourous stirring in the primary before adding the other clearing agents and racking back to carboy. So moving the bentonite to the start removed one racking for the winemaker.
Steve
