There's a few things to keep in mind.
First is pushing the boundaries. It's like Formula 1 or really any high end sports car. They're special things that people buy because they have a love of them, not because they're fooling themselves into thinking they'll always be going 250mph. From the manufacturer side its PR and pays for the development which eventually trickles down to their more consumer grade products. Well its the same thing with Lenses. Most don't need faster than f/1. But its a thing of beauty. And the manufacturer's know this and put their highest grade of research into these lenses. Even if you never once shoot a Noctilux at wide open its still an incredible lens for sharpness, contrast, colors, build quality, etc.
The second issue which perhaps more precisely answers your question is there are cases when the low light capabilities are not only nice but really required. Caves, Underwater, Night are three such cases.
Famously Stanley Kubrick used a f/0.7 lens to film parts of Barry Lyndon illuminated only by candlelight:
Video discussing the Cinematography of Barry Lyndon: https://www.youtube.com/watch?v=FmSDnPvslnA