Steam has announced the launch of Steam Remote Downloads. This feature allows you to start downloads in your Steam client from a web browser or Steam mobile apps.
The company spoke about this new feature:
To kick off a remote download, simply visit your Games list on the Steam Community site, which can be accessed from your own profile using a web browser or the Steam mobile app on iOS or Android. If logged into the latest update of the Steam client on a PC or Mac, there you’ll see which games are currently installed, and have the ability to install others from your library. Or, seamlessly complete your online purchase with remote installation: When a remote session is detected, the Steam store will ask whether you’d like to download your new purchases immediately.
The idea is to let gamers start their downloads after purchasing games online from their office. As a result, they would have their games downloaded by the time they reach home! Gamers would need to leave their machines on at home with the Steam client running for this to work.