i'm pretty sure these are the analog to framegroups in models and sprites, basically client-side looped animation.

based on the engine code, it looks like glquake has pretty half-assed support for skin groups, for example every skin is given 4 pointers to textures, and almost all the time the 4 pointers are to the same texture, but when loading a skingroup it gives them unique pointers. If you have more than 4 skinframes, it drop some.

So effectively you can only have 1, 2, or 4 skinframes in a group in glquake. If you have 3, it will animate 1, 2, 3, 1, 1, 2, 3, 1... and if you have 5+, it will animate (e.g.) 5, 2, 3, 4, 5, 2, 3, 4...

Also, not sure if the model format supports intervals for skingroups, but glquake clearly only does 10Hz animation for them.

My guess is there's one model somewhere in the mission packs using this feature, and that required them to add this minimum of support to glquake when they wrote it.