I think that the main use comes from when you're not at your own computer. If you're at work or a friend's house, it'll be far simpler to just drag the file into the browser than to install dropbox.

I'm in that situation about four times a year and this will be a huge time saver compared to the old system.

It's really nice if you're on a friend's computer and want to copy a file to your own Dropbox account.