This service converts your email into an image. You can save a copy of the image, and keep it forever, even though the mail will disappear. It's novel, but I'm not sure I see use-case for it.
Think about gchat's off-the-record feature. Anyone can copy and paste an entire conversation, but the thing is, most people aren't motivated enough to copy every single one, so you have an added element of safety. Further, everyone I use ghcat off-the-record with is someone I trust - I just don't want a copy of the chat sitting in their email.