It is my understanding that hardware accelerated video encoders (as in the fixed-function ones built into consumer GPUs) produce a lower quality output than software-based encoders. They're really only there for on-the-fly encoding like streaming to twitch or recording security camera footage. But if you're encoding your precious family memories or backing up your DVD collection, you want to use software encoders. Therefore, if a hypothetical software h264 encoder could be faster on the GPU, it would have value for anyone doing not-on-the-fly encoding of video where they care about the quality.

One source for the software encoder quality claim is the "transcoding" section of this article: https://chipsandcheese.com/i/138977355/transcoding