FS is an abstraction, a common interface for file systems.
It allows me to provide access to an archive (zip, tar) or database or network (S3, HTTP ...) backed file system.
Thus exporting FS as a FUSE filesystem allows all those FS implementations to be accessed as a real file system,
eliminating the double implementation (say, if you implement a HTTP backed FS, you can use it as a FUSE fs, don't have to reimplement it with a fuse lib).
You're right, exporting a file system backed FS as FUSE is moot.
But exporting the embed.FS or database backed FS as FUSE allows ordinary programs, user, scripts to access those files.
FS being read-only is a bless and a curse at the same time: simplifies implementation, but deflates the usefulness.