One possible reason to do just that is a rather simple IMAP-less setup of several email clients and occasionally-used webmail. If you set each mailclient to fetch but not delete, you can receive the same mails on more than one client (without having to use IMAP, which works a lot better when you have a constant connection to the net) and store them locally. In this scenario you very rarely use webmail – only really if you’re on the road and don’t have your usual client with you; in that case having left the mails on the server lets you answer or read mails you had received before but now need on the road.
IMAP rocks, but there are still clients that don’t do offline storage of IMAP-folders all that well (and some external webmailers have “collection” agents that only work with POP3), so there are some legitimate use cases for this. Unfortunately, many mailservers get bogged down rather heavily on access (and even submission, though there REALLY should be no reason for that) when folders exceed some arbitrary limit. I recently had the pleasure of accessing a mail folder containing just over 120k messages using IMAP … That was fun. Good thing the underlying filesystem handled that maildir well, though it still took several minutes before I received even just the messagecount (Hint: clean out your positive-virus folders every now and then)