How is Google Stadia different?

How is Google Stadia different from Nvidia's Geforce Now and Valve's Steam link / app? Its very dependent on your internet connection even tho Google is making some broad claims. The listed games that are available are pretty old. It's also running off a Chromecast Ultra ???? I dont get so why is it so expensive ...do you need the chromecast if only want to play on pc / phone? I thought the price was good at first but maybe its too much and you have to pay for service and buy games?

I have a Nvidia Shield which i can do basically the same thing they are advertising and actually more, I can stream games from my PC and I can stream games thru their service Geforce for games i already own. I dont see the advantage, are they going to let use games from Steam, Epic, UPlay or Orgin. I guess time will tell. I'll  keep updating as news becomes available.