But why is Steam used by so many? Is it because they want to use it, or because they are forced to use it? (as CA is forcing ETW customers to use it)

Valve's strategy with Steam is the same as Microsoft's was with it's OS - to force-bundle the software with purchases of other products.

If Steam truly provided benefits to users, Valve wouldn't need to pay off game developers to bundle Steam with their products and force game buyers to install it and set up Steam "accounts". There's no need for every ETW buyer to set up accounts with Steam, when the multiplayer version of ETW will not even be in beta by the time the product is released (unless Steam is providing some sort of DRM)