Traditional solution was imperative file transfer - there's always some way to order a computer to transfer a file somewhere else. There's a ton of issues with this since computers are not always turned on, they're not always on networks, file transfer fail, in many cases you want to edit the same resource from different computers at different times and still want just one canonical representation of it.
Here comes Dropbox (and similar Cloud-based solutions) - its biggest innovation is replacing the imperative paradigm with declarative file transfer. You link a directory like ~/Dropbox with your account, you add, edit, and delete files and directories within it any way you like, and without your effort to babysit all that Dropbox takes care of it to make sure your ~/Dropbox directory everywhere else also reaches such state.
This solves a lot of issues like:
- You want to keep .dotfiles across multiple machines in some single versioned repository - you can solve that with Dropbox and some symlinkery (or a private github account and some symlinkery).
- You want photos you make on your camera available on your computer - at the phone should do the uploading whenever you're on some wifi, without your intervention. Dropbox completely solves that problem.
- You play a game like Cockatrice on multiple machines and you edit your decklists on multiple machines. Manually sorting out which decklist is the most recent one is pretty annoying - but Dropbox does that for you.
- A simple todo.txt file is pretty awesome if you're moving between computers a lot, even if it's obviously not a full GTD solution. In the past I did silly things like making gmail drafts for that, which I then accessed from multiple places.
- You want to write a quick note for action later, but you're somewhere outside with no paper? Simply open Dropbox directory on your phone, create somefile.txt, write whatever you wish there, and you'll get it on your computer for later access automatically. There's no way to forget about it.
- For that matter simply transferring files between computers not on shared network can be a big pain in the ass. For small files people just send emails to themselves with such files as attachment - and it's another problem Dropbox solves completely.
- you only ever use 1 computer and 0 smartphones (pretty unlikely if you're reading this blog)
- you use another Cloud storage service like Google Drive (with drivers installed on all your boxes)