The buttons appear in the subject line in your inbox, so you can click them without opening the mail first. I think the use cases are fairly limited, but I can

The buttons appear in the subject line in your inbox, so you can click them without opening the mail first. I think the use cases are fairly limited, but I can think of a few interesting ones (for example, bugzilla notifications could link directly to the bugzilla page, mailing lists could have a "manage subscription" button, etc)