IS Java 8 update 431 necessary for os 10.13 on a Mac Pro
I currently have update 391 build 10
Mac Pro, macOS 10.13
Java is not required to run MacOS in the usual way. Some consider it a security risk, and simply won't have it on their computer.
But if you wish to run Java Apps, ether directly on your Mac or inside a browser, then Java would be needed for that, and having the latest version for your particular computer would be recommended.