For people who are already throwing money at Valve on a monthly subscription and might otherwise have bought something from Sony or Microsoft, or more likely, will also buy something from Sony or Microsoft.
The consumerist mindset accepts this device.
There's no monthly sub for Steam.
All games are free then?
Very few games are free. Otherwise they range from $0.99 to $90-ish. Sometimes I get a bunch from Humble Bundle deals. Sometimes I buy something that looks interesting but never play it.
How's that relevant?
There's no Steam subscription of any kind to access your library. Whether you're spending $0 on average year, or $300 -- you get to access your Steam library the same.