It serves at least two purposes. One is the beginning of clearing, bentonite pulls down positive charges. Second is, the yeast like to have something to rest on to bud, as in produce more yeast. The bentonite gives it a place to do that.
During fermentation the yeasties grab the bentonite. When the CO2 is formed they float to the top and release the C02. Then they sink back to the bottom. So the bentonite help corculate the years.
It should be done for all kits and I added some to most wines I make. I don't added any, if I am using grapes. There are plenty of things for the yeast to bud on.