In the first place, IMAP servers, unlike POP servers, can keep track of flags for each message, indicating things like which messages have been read, forwarded, or replied to. With POP, your local email client has to track this information. That means if you download a bunch of messages on computer A using POP but leave them on the server, and then switch to computer B, you can still download the messages again, but they'll all appear to be unread - computer B won't know which messages you've seen, filed, or otherwise dealt with, because
that information isn't kept on the POP server.