You're right, GLSL can be a bit confusing at first, especially swizzling, and the idea of writing your kernel only once then relying on the input data to drive your logic. But once you get used to it, it's very practical for writing complex stuff in just a few lines of code, it's really fun, and that's what you see a lot in Shadertoy.
I got the idea to write the library when I wanted a simplex noise function in C++, implemented with AVX2 for performance reasons (because why not). There were a lot of public HLSL/GLSL implementations that were concise and fast on GPU, some of which I had used for years for my own needs, but rewriting the whole code in plain C++ would have been a hassle, and i would have to do the same for every GPU code i came accross in the future, so building a wrapper seemed like the most efficient approach.
So in the end, the library is mostly aimed at people coming from the GPU world who want to keep most of their habits. But yeah, there is probably very few use cases for it.