use case: The Nomad

Alice is always on the move, often with her trusty netbook and a small handheld terabyte USB drive, or a smaller USB keydrive. She has a server out there on the net. She stores data, encrypted in the Cloud.

All these things can have different files on them, but Alice no longer has to deal with the tedious process of keeping them manually in sync, or remembering where she put a file. git-annex manages all these data sources as if they were git remotes.
more about special remotes

When she has 1 bar on her cell, Alice queues up interesting files on her server for later. At a coffee shop, she has git-annex download them to her USB drive. High in the sky or in a remote cabin, she catches up on podcasts, videos, and games, first letting git-annex copy them from her USB drive to the netbook (this saves battery power).
more about transferring data

When she's done, she tells git-annex which to keep and which to remove. They're all removed from her netbook to save space, and Alice knows that next time she syncs up to the net, her changes will be synced back to her server.
more about distributed version control