Yes, so this is useful where deployment is "manual" in the sense that it is not a mere git push. It is bad practice to have git data lying around among otherwise public files, so you have to either set up git hooks on the public server, or fine-tune your public server access rights. Both require (slightly) more work than simply not pushing the git data in the first place.
> It is bad practice to have git data lying around among otherwise public files
Usually I have source files in a “src” subdirectory and assets in an “assets” subdirectory and configure the web server to serve from these two subdirectories.
So the git repo root is one level above anything served by the web server.
have git data lying around among otherwise public files
There are almost no public files on a server these days. It's mostly backend code generating output by combining data from databases and templates. So since it's already all non public stuff, I don't see a problem having the .git folder there too.
