Power user replacements for OS X default apps 15 years ago
I used to do all my backups locally to a NAS and various USB drives. Dropbox allowed me to sync between multiple computers running different OSes which is not an easy thing to do normally. I've found myself syncing .emacs, .ssh/config and other customizations across systems and it's so much easier.
Another bonus for Dropbox is the ability to share folders with other users. It makes sharing docs so easy because everyone sees the same version.