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.1 It’s great for installing the OS on multiple Macs, because you don’t have to download the ~5GB installer onto each computer, and it serves as a handy emergency disk if your Mac is experiencing problems. 2 Here’s this year’s version, for both macOS High Sierra (10.13) and macOS Sierra (10.12), of my annual how-to guide.

It’s never a bad idea to have one of these.