[IMAP] is an application layer Internet protocol […] that allows a local client to access e-mail on a remote server.
Having never used IMAP myself before, this sounds to me like you can now use your Gmail account without ever logging into the online web interface. Using the IMAP commands, you can send, delete, archive and spam your Gmail directly from compatible clients such as Thunderbird. Now even though I’ve fully switched from Thunderbird, I can still see the smartness in this. Your Gmail account just became a funnel for all your other mail accounts. One account to rule them out. You pick your interface. Neat.