Why?
Sometimes you need it. Recently for installing tensorflow with python. Which requires python’s numpy uninstalled from MacOS’s default installation.
Step 1
Reboot your system and press Command-R while booting. Just as you hear the startup sound.
Step 2
In the recovery screen, find and open command console from menu.
Step 3
Type
# csrutil disable
# reboot
You are done.
Remember to revert back once you are done.