Never store gerbils up your ass, we went over this!

There's a lot more that Buttplug.io supports. It's an entire open source framework that others can build stuff onto to control sex toys. As for why these? Eh why not? Single player games don't exactly require the consent of other players if you're getting your rocks off to game controlled sex toys, and multi-player games should only be used if you do actually have consent.

Specifically for ultrakill's officially supported mod, that's just the kind of thing they'd do. They also made body pillow covers of Gabriel and V1 as official merch. And yes they are in those kinds of poses.