Where kits are concerned, glycerin is more for body/mouth-feel. Kits, even some nice ones, can be thinner than their commercial counterparts. Gum Arabic or glycerin can improve mouth-feel, as long as it is added carefully, so as not to get so much it affects the taste. Too much and it can create a sweetness and/or a slimy feel.
Glycerin can take the bite off a sharp tasting wine by creating a sense of sweetness. IMO, there are better products, like Biolees, which can better accomplish this.
As was mentioned, most young wines have a tartness, that will go away over time. I suppose a person could take the edge off that tartness with glycerin, but the better way is to let it mature on its own.
