like offlineimap, fetchmail is used to pull emails from a remote mail server and deliver them to a local mailbox or forward them to another email address
so this is in effect a backup
fetchmail supports IMAP, POP3, ETRN, ODMR, and others
OfflineIMAP supports only IMAP
fetchmail doesn't emphasize two-way synchronization as its primary function
OfflineIMAP emphasized two-way synchronization
for a backup this shouldn't be necessary anyway
fetchmail is more for simple fetching, backing up
OfflineIMAP for keeping mail accounts synchronous
if you want to periodically download emails from your email server and store them locally for backup purposes, fetchmail can be a suitable choice
easily manage a server for your friends, association or enterprise
user-friendly interface for managing various services and applications on a server, allowing individuals and small organizations to host their own websites, email services, file storage, and more
takes care of Mailserver, Firewall, Domain and connection to apps