You can do it faster – All it takes to reinstall macOS on a computer is to simply plug in the USB drive and get the mac to boot via external source (your thumb drive). You can do it offline – Installing macOS via USB will allow you to do it offline meaning that you wouldn’t need to wait for the re-downloading of the entire macOS installer, which could take a while depending on your internet speed. Well, if you do want to install macOS on your computer, I would suggest you to do it via USB. As a mac user, there may come a time when you’d want to reinstall macOS on your computer- say something goes wrong with your machine or you just want to have a fresh start.