Google Help Center guru has a good post on the Gmail blog about some of the differences between the supported email protocols offered with your GMail account, IMAP (Internet Mail Access Protocol) and POP (Post Office Protocol).
Clearly in the post IMAP is the preferred protocol, and I couldn’t agree more. However, there are also a few other reasons not mentioned (source):
IMAP protocol advantages:
-Can manipulate persistent message status flags.
-Can access and manage multiple mailboxes.
-Can support concurrent updates and access to shared mailboxes.
-Suitable for accessing non-email data; e.g., NetNews, documents.
-Can also use offline paradigm, for minimum connect time and disk use.
-Companion protocol defined for user configuration management (IMSP).
-Constructs to permit online performance optimization, especially over low-speed links.
A few years ago POP was more common however with several great (free) email clients out there that offer both, IMAP is likely your best bet.