If you leave mail on your mail server, and are fetching mail over a WAN, with IMAP you only fetch messages with a higher ("newer") unique ID on each connection.

If you leave mail on your mail server, and are fetching mail over a WAN, with IMAP you only fetch messages with a higher ("newer") unique ID on each connection. With POP, you scan the entire map of unique identifiers each connection. With a large mailbox and a relatively slow WAN connection, this creates a significant overhead with POP. IMAP pretty much supplanted POP. Most people nowadays use IMAP, unless they have some specific reason to use POP.