Sometimes, even if you know you're starting with somewhat suboptimal performance, the ability to use CPU code you've already written and tested on the GPU is very valuable.

Many years ago (approx 2011-2012) my own introduction to CUDA came by way of a neat .NET library Cudafy that allowed you to annotate certain methods in your C# code for GPU execution. Obviously the subset of C# that could be supported was quite small, but it was "the same" code you could use elsewhere, so you could test (slowly) the nominal correctness of your code on CPU first. Even now the GPU tooling/debugging is not as good, and back then it was way worse, so being able to debug/test nearly identical code on CPU first was a big help. Of course sometimes the abstraction broke down and you ended up having to look at the generated CUDA source, but that was pretty rare.