The "usefulness" (whatever that means) isn't really the point. The point is that the .stp file is the native format that is consumed and produced by cad software and the .zip is not. Just as github previously introduced a tool to show the geometric differences between two 3d models, should someone produce a tool that can compare two revisions of a .stp and show meaningful information, then having the native format in the repo would be quite useful compared to trying to dig it out of zip files.

Furthermore, git natively compresses the data in a repo so compressing the file isn't even that useful in the first place.

I do admit that I'm just ranting and none of this matters that much. It would probably be best to just file an issue asking them to expand the file in the repo.