It gives you more control over when and what Google can do on the device. If you run Google software with network access you have to assume they can track you, but having it sandboxed minimizes what it can do while running and gives you more control over what it does (e.g. you can deny it network access entirely whenever you want).

Last I saw they have something like 900,000,000 active users with around 50,000,000 on a paid plan of some form.

This is not correct. You can use Windows without a Microsoft account.