For OMEMO to function smoothly it wants permanent storage for Publish/Subscribe functionality (XEP-0222) on the server. The feature predates OMEMO and was designed for various other purposes, such as storing a contact roster or profile picture on your account. OMEMO works without XEP-0222, but then the clients are forced to "meet for the first time" every time they connect and want to exchange encrypted messages. OMEMO in itself happens entirely on the clients.

I see. Thanks!