It's very useful for when you leave your house keys, public transport card, and office keys in your backpack, which you left in your partner's car, who has driven said car to his/her work. And you can't lock the house door without a key. And you need to take a bus. AND you need to access your office. So, guess what? You're working from home today, and you better hope you can remotely access all your documents!
I use Dropbox, but there are other cloud storage services out there. If you need more than the 2GB free storage space that Dropbox offers, then Mediafire and Mega look promising. Although the free accounts they offer cap the upload file size at 200 MB and 300 MB respectively.