Earlier quoted context omitted.
The difference is, if Dropbox is down tomorrow, I still have all my data backed up on my computer(s).
You can have that with any provider: just don't delete the data after you've uploaded it.
With Dropbox, I only have to worry about maintaining one place, the Dropbox. It syncs across all my devices.
Without that, I'd have to sync files manually whenever I delete/rename/change them... I hope you see that it's not the same.