Mail is the business record people most often realise too late they have lost.
WHAT IS AT RISK
Hosting backups have finite retention. A mailbox deleted six months ago may predate every copy we hold. Cancelling hosting removes the mailboxes. A compromised account can have mail deleted deliberately. A POP3 client holding the only copy is one hardware failure from losing everything.
HOW TO BACK IT UP
cPanel > Backup > Download a Full Account Backup includes mailboxes, but not every host restores those cleanly elsewhere.
The more reliable method: connect each mailbox in Thunderbird using IMAP, create a local folder, and copy everything into it. That produces a copy on your own computer that you control.
Do this before cancelling hosting, before migrating, and periodically as routine.
HOW OFTEN
Quarterly for most businesses. More often if correspondence is contractually important.
WHAT TO CHECK
That the copy actually opens and contains what you expect. An untested backup is an assumption.