What does this button do?

[MrBehemoth]

The engine seems to lump static objects together into groups when you load a map. You can see them with inspection mode.

I believe those groups are PVSs (Potentially Visible Set) used to calculate occlusion culling. If that's the case, this button in the editor would be for making your own occlusion groups.

How would that be useful? Well you could use it to boost performance by restricting limiting groups by line of sight, but the engine seems to do a decent job of this all by itself.

Disclaimer: This is completely guesswork and I've not tested it in the slightest.