Because some mail hosts (ISPs, universities, whatever) don't provide IMAP support. duh.

With thousands of messages in over 20 different folders, I'd rather keep my archived messages on my laptop rather than use up my university disk and mail quota. Keeping all my archived mail on my own machine also gives me a greater sense of privacy and security.

You can still read your IMAP mail offline in Mail.app and it DOES save a copy to your hard drive.

Either way you can export it as you wish from IMAP, or if you ONLY have POP3 you can leave the POP3 mail on the server.

If your big concern is security then this will let you have mail in TWO places which is better than one. Probably a lot more than two places as your work/university should be doing multiple backups of the data. This is much more secure than just on your friggen LAPTOP's hard drive. One drop and all your data is gone!

Frank