I generally use that if, you know, what I'm sending isn't already on the Internet. Like a document I just made. Or a paper I just scanned. Or a music file I just recorded. Or lots of other things.
Or maybe even a link to where to find the particular object on the Internet, in case it is already there. I mean, if it takes a long time to find it, why not send myself the link in email, to access it quickly from anywhere... Isn't this the point of favorites/bookmarks anyway? I am aware that the idea of "universal favorites" has been invented, but why would I need a program or toolbar or widget for something that could be done in 20 seconds by sending myself an email?