Why? POP access downloads your emails to your desktop, which works just fine if you only use one device to check your email. If you like to check your email from multiple computers, the occasional cell phone, and maybe a PDA... IMAP is what you need. IMAP syncs your email rather than just downloading it.
Not only is IMAP access a very nifty feature for a free email program, it's a feature that boosts Google's cell phone strategy and makes Google Apps better able to compete with Microsoft.