It's hard to pin down a single use-case, because there are so many situations where it turns out to be useful.
Maybe I wrote a document on my laptop, which I don't have on me and I forgot to send it. I always have my phone, so I'm in never in a position where I don't have access to that document and can't complete that task.
Maybe I have to wipe my phone or I get locked out of it, and I don't have to worry about losing anything and I don't have to put in any extra effort into backing things up (with the risk of forgetting something).
