The purpose of unattended-upgrades is to keep the computer current with the latest security (and other) updates automatically.

If you plan to use it, you should have some means to monitor your systems, such as installing the apt-listchanges package and configuring it to send you emails about updates. And there is always /var/log/dpkg.log, or the files in /var/log/unattended-upgrades/.

As of Debian 9 (Stretch) both the unattended-upgrades and apt-listchanges packages are installed by default and upgrades are enabled with the GNOME desktop. Rudimentary configuration is accessible via the "Software & Updates" application (software-properties-gtk).

To install these packages, run the following command as root:

# apt-get install unattended-upgrades apt-listchanges

The default configuration file for the unattended-upgrades package is at /etc/apt/apt.conf.d/50unattended-upgrades. The defaults will work fine, but you should read it and make changes as needed.

# editor /etc/apt/apt.conf.d/50unattended-upgrades

This section controls which packages are upgraded:

Unattended-Upgrade::Origins-Pattern {

// ...

You should at least uncomment the following line:

Unattended-Upgrade::Mail "root";