I think the gain is in footprint size and noise levels. I have a Sonnet eGPU box, and it is pretty massive, nearly the size of a microATX case. It also has a fan on the box and the fan of the GPU cooler itself.

This design looks a lot smaller, and I'm sure it is a lot quieter as well. If you don't want those things and do want upgradability there are plenty of other options available already.