A: If you have installed Linux or Windows via Bootcamp, yes. Native ChromeOS? Only Android ports or via Steam Link streaming.
In many parts of the world, high-speed internet is not a guarantee, nor is unlimited data. Downloading a 70 GB game like Call of Duty: Warzone is simply impossible for some. A 500 MB game downloads in minutes (or even seconds on a good connection), offering instant gratification without eating up a monthly data cap.
A: Yes. Most Steam games allow moving installation folders. For GOG games (DRM-free), just copy the folder and run the .exe.