How many people do you think are actually interested in running the client in browser? I have Folding@Home on about 5-6 machines and I can't imagine a situation where I could run a web client but not a full client.
It isn't for you. It is for the person who can open a tab, but doesn't want to run a full client. Installation, sandboxing, laziness, etc.
A university or corp could push a tab open script to thousands of desk tops, while they wouldn't push Folding@Home client.
It's not an incredibly common use case, but many computers exist simply to show a dashboard or visualization. It would be excellent to say "drop this script tag on your page" and you instantly turn that Mac Mini into less of a waste.
A fair number of people are already running the Chrome NaCl client in their browsers but it is CPU only.
Those are pretty woefully underpowered for folding..
If you just care about GPU compute and not much about the shape of the API or it's overhead, then something like OpenGL ES 3.1 Compute Shaders or WebCL would be much easier to reach than WebGPU, technically.
Mozilla won't support WebCL in favor of compute shaders, but that announcement has been over 4 years ago and there still is no general compute in browsers: https://bugzilla.mozilla.org/show_bug.cgi?id=664147