There are a bunch of different use cases we have thought about.

Perhaps my favorite is a mirrored workspace. An app like Asana could use the Shared Folder API to create a Dropbox shared folder that mirrors an Asana project. As you add or subtract users, those changes affect both. Similarly files added via the Dropbox folder appear in Asana and files added in Asana are instantly put into your Dropbox. A video conference has similar concepts.

But there are tons more and we'd love to hear ideas from other folks!