I'd think non-appending writes are quite rare in practice, other than databases. Even when the application is logically overwriting data, in other kinds of programs it's almost always implemented as writing to a new file + an atomic rename, not in-place modification.

Most programs either write a full file every time and replace the old file by a single move or append to an old file. Writting in the middle could happen in a program writting to some kind of archive or disk image. There is probably a whole group of programs that do this I'm not familiar with, but I'm pretty sure of my first sentence.

well yeah, but there's a lot of things FUSE makes easier. no need to implement a client library, no need to write some custom wrapper or rsync thing to sync files to the bucket or bucket to local system, etc. it won't work for every app but for the ones it does support it saves a ton of extra work and maintenance.