This is useful for trying to recover from an unresponsive system. However, by default, Ubuntu disables the key to gain raw keyboard access.\
Sometimes part of the issue is that X/Wayland display servers have crashed due to bad graphics drivers and keyboard input isn't processed anymore, whereby changing to another console, e.g. ctrl + alt + F1 doesn't work. Since the default disables keyboard control, alt + sysrq + r, can't help salvage the situation. Hence the suggestion to at least enable system request raw keyboard control when trying to recover a system.
Ubuntu 16.04 LTS default value: