Why do we need app stores and streaming game platforms?

Aside from a storefront where people directly see your competitors beside your product, and the reach that those stores have, what value to app stores and game platforms (App Store, Play Store, Steam, Epic, etc.) bring to software developers and publishers for the large cuts of revenue they get? You still have to run ads to reach your market, so why is it not normal to direct to your own website, purchase process, and download/install? (walled garden hardware aside, that's it's own poison)