It also provides some added value like achievements, basic social media in forms of forums and chat, some (somewhat broken at times) controller handling, some bug reporting, cloud save handling, voice communication handling, even some space to host game related content. On the whole, more value added than taken by paltry ads. The bigger value minus is the "Steam DRM" - requirement for an online connection to use the downloaded application. And providence help you if you post inflammatory comments under an account that owns game access licenses. You will lose that.
The other ones don't do it quite as effectively, esp. the suggestions part. Being a game library and download service is not what makes Steam in particular good, though it is an essential part.