So, why IMAP?

Simple, all my mail on all my clients.

If I use POP and have multiple clients, then I do not have all my mail in one place. POP downloads the email to the device that I viewed it on. It is no longer available to any of my other devices.

Why would I NOT want to use IMAP? 

Well, if I am a WebMail user.  My WebMail folders and my IMAP folders are not the same. 

Also, there are email quotas. Pruning is done for POP, but not for IMAP.