Suzy-Q cable is used to low-level interact with newer chromebooks firmware, you can for example disable firmware write protection (previously it was done with a screw), debug hardware and unbrick chromebook

you can read on it a bit more in official Google docs: https://chromium.googlesource.com/chromiumos/third_party/hdctools/+/HEAD/docs/ccd.md

you can play on multiplayer just fine, as long as server have enabled offline-mode

For minecraft you can just use a offline mode in pretty much every 3rd party launcher, like SKLauncher