Originally, Gnome was using xscreensavers. I like the Matrix one the most. At one point around 2011, the Gnome team decided to abandon xscreensaver, actually, all screensaver at all. That is a very brilliant, bold move.
The screen saver was invented 30 years ago, before LCD screen was not invented. The CRT screen has "burn-in" issue, if it displays the same image for too long. That is shortening the life span of the screen. Now things are changed, and you don't see CRT screen any more. LCD screen ( and LED screen) still have a little bit of "burn-in" issue, as serious as "life" issue. The screensaver activates each cell of the LCD/LED screen when the computer is idle, and that is as bad as burn-in issue.
So in the LCD/LED age, the best way to extend life span of a screen is to turn to black, or even turn it off, when it is idle. Exactly what the Gnome team is doing.
Of course, in Linux world, you still have your free will. You can easily remove the gnome-screensave, and install xscreensaver by (as in Ubuntu):
sudo apt-get remove gnome-screensaver
sudo apt-get install xscreensaver xscreensaver-data-extra xscreensaver-gl-extra
But if you are like me, you like keeping the system clean and neat, you can just go to the System Settings -< Brightness and Lock, and select how long of idle before the screen turns black, using the gnome-screensaver.
The article is about Linux Gnome screen saver. The same principle can apply to Windows or any other OS too.
It is always a good practice to check the "Require password when waking from suspend", to keep the computer locally secure.