Information
Auto Update verifies that your system has the newest security patches and software updates. If "Automatically check for updates" is not selected, background updates for new malware definition files from Apple for XProtect and Gatekeeper will not occur.
http://macops.ca/os-x-admins-your-clients-are-not-getting-background-security-updates/
https://derflounder.wordpress.com/2014/12/17/forcing-xprotect-blacklist-updates-on-mavericks-and-yosemite/
It is important that a system has the newest updates applied so as to prevent unauthorized persons from exploiting identified vulnerabilities.
Solution
Run the following command to enable auto update:
% /usr/bin/sudo /usr/bin/defaults write /Library/Preferences/com.apple.SoftwareUpdate AutomaticCheckEnabled -bool true
Impact:
Without automatic update, updates may not be made in a timely manner and the system will be exposed to additional risk.