Do I need to create a device with CephFS or can I just make an OSD and mount it as an RDB storage in Proxmox?

Question

Proxmox

Good afternoon! I'm learning Ceph.
Task:
OS: debian buster 10 (proxmox ve)
Three servers in a rack, two of them are connected via FC to msa2040.
Inside, each server is allocated 2TB for separate LUNs (each server has its own device). It is
necessary to make a common storage and space for virtual machines, etc.
In the test version, I did this:
Created osd, raised monitors and osd daemons on two servers (out of three)
But then I saw that it was possible to make the third server in the rack a metadata server and file CephFS.
How necessary is it?

Class OSD: HDD
Type: Bluestore

Now storage in which everything works on proxmox: RBD