I'm not sure, but I think it's targeting only console games. It feels like a gigantic value-add for Chromecasts first (which generally live in living rooms with more controllers than keyboard/mice), which also happens to be capable of working on phones/pcs using the same streaming technology so it's also available there.

Here's the list of the launch day games [1] -- I'm not familiar with them all, but even ones like Farming Simulator say they have full controller support on their Steam page. I wouldn't be surprised if Stadia required a controller at least for a while, but eventually got PC-only games that could use a mouse/keyboard.

[1] https://www.cnet.com/how-to/how-to-play-games-on-google-stad...