Dan Frakes:
I’ve long recommended creating a bootable installer drive—on an external hard drive, thumb drive, or USB stick—for the version of macOS you’re running on your Mac. It’s great for installing the OS on multiple Macs, because you don’t have to download the ~6GB installer onto each computer, and it serves as a handy emergency disk if your Mac is experiencing problems.
Here’s this year’s version of my annual how-to guide. The procedure has changed slightly this year due to a change in Apple’s createinstallmedia tool.
Perfect timing. I’m downloading Mojave as we speak.
As an aside to the middle part of that, that’s one benefit of having a dedicated OSX server machine in your house, if you have lots of Macs.
It can handle aggregating update downloads for you, as a central repository.