While you are typing the text in the text boxed is saved regularly, so if you leave the page without actually hitting cancel or send it will stay saved as a message draft. This can be clicked to continue writing on the message later on.
Basically it can be used to take a break while typing a message to continue it on the next day, but my favorite use for it is to not loose the entire message if the browser crashes, or if you by mistake close the page or your internet connection is lost.
I'm not sure if gmail can save more than one draft, but I think it would be enough to just have the latest written message saved until it has been saved or cancelled, or another message is written, without choosing to re-edit the draft.
