The purpose of the tag is to help either find images with a lot of versions (if you like that sort of thing) or to hide them in order to have a less cluttered image list - especially with a complex filter that hides images tagged both multiple variants and alternate version, then you’ll see only the first image of big sets of images, but see all of them when there are only a few.

This compensates for Derpibooru’s lack of a “pool” feature.