The usecases are related but difference -- these are block storage solutions, not filesystem level solutions. Some technologies work on multiple levels (Ceph), but others do not.

Isn't NFS generally served by a single machine? I don't have anything inherently against NFS, but creating a SPoF / a box that can't rebooted doesn't sit well with me.

There are guides for HA NFS servers. I don’t have experience though. I assume that it requires shared block storage.

Any case where you need horizontal scaling or redundancy though data replication. So lots of cases.

ceph(and the others I assume) solves the same problem that raid solved, but using the network as a transport layer(rather than the computer for raid). this generally means it is very easy to scale.

So the answer to your question is that your question was wrong, you are comparing different layers.

you can actually access ceph over nfs, however usually you will try to use a ceph aware access method, ether the ceph filesystem driver, or the S3-like library depending on what your application wants.

From personal experience, if you want to run some software that uses SQLite or similar...