Oh. I guess that's a use. I had no idea what it did since its existence was only pointed at by one line in the news feed. I just assumed it was like d3d_set_hidden. So what DOES it do, then?
I'm afraid I'm not too knowledgeable on how exactly it works technically, but the effective difference between this and turning off hidden surface removal is that this only affects the interaction between the primitives drawn while it's turned off, they still interact normally with the rest of the scene. So in my screenshot I'm using GMs inbuilt particle system (which unfortunately means it's flat) drawn transformed to face the player and with zwriting disabled around the part_system_drawit() command. If I had turned 'hidden' off instead it would probably have drawn behind or in front of everything else.