> I think of this as the very essence of what Dropbox does, are there really very many situations where local copies would be unavailable?

I can think of a lot of situations where the local file is a gigabyte or more and I'd rather not sit around letting the web form upload it.

With this, I can submit immediately, and the site can handle the heavy lifting on their backend. I get to wander off and do something productive.

One bonus is that the transfer from Dropbox to a web service is going to be way faster than from your computer to that same service.

Additionally, the user can navigate away from the page, close their browser, lose internet, etc. and the transfer will still happen.

I mainly use Dropbox to only keep copies of things I need right away (small footprint mentality). The selective sync is fantastic for letting things be out of sight until I need them. Backups are great for this concept, I can keep tons of photos in my dropbox account but don't need them hogging my drive space. Enter the benefit of server > server & not needing a local copy of it.

What it's useful for is web applications that don't want to deal with user and file management.

We don't want to have to do a UI to manage files for every cloud storage provider. This is something Google Drive is well in the lead with currently.

While syncing files is the main attraction for me with Dropbox, there are other useful features too. You can set which folders sync with other machines or only upload to Dropbox, as well as generate links for files in your Dropbox.

mobile/tablets, chromebooks, shared computers, friend's computers... there's plenty of instances where you don't have a dropbox client available.