it downloads shit the the demo requires (engine, etc) and leaves out stuff you won't need (maps and models not used in demo, etc) then if you choose to buy it down the line it's quicker to download :3
Yeah merchuke is right. Also dont this steam works this way, but shareware titles have (usually) all the content you need. However most of it is block by a passcode or serial key.
If you decide to take the easy way out, at least do it some interesting way, like 5 fishing magnets and ropes... Run out to the space x rocket as it's about to take off.... Ohhhh add some snow skiis and skii into to space!!!