The real purpose of screensaver and its advantage benefit for PC user, CRT monitor burn in, and modern PC
The Real Purpose
The screensaver was created to prevent burn-in from phosphor that was contained in CRT and Plasma monitor by changing the picture or moving pattern on the screen when the computer isn't being used. Because when the computer screen is displaying the same image for a long time, the phosphor layer in the monitor will be damaged (Phosphor layer helps to produce a quality image), and that is the origin of the "screensaver" name. Modern CRT monitor has a lower burn-in probability than the old CRT because of more advanced phosphor layer technology. And unlike CRT, LCD screen does not have a burn-in problem, because the image wasn't produced directly with phosphor.
Safety
A modern computer user can still get the benefits from the security feature of the screensaver. You can configure your screensaver to ask for a password after the screensaver appeared or when your computer is idle for a specified time, and prevent anyone to access your computer when you are not around.
Decoration
Now, most people use the screensaver as a decoration and also for entertainment. Like changing how your computer appears to look more beautiful with some waterfall, and butterfly flying around, or classic, modern, and futuristic digital clock screensaver. Of course, you can download all of those interesting screensavers, for free, from the internet.
Activate Task in the Background
One of the popular usages of the screensaver is to activate background tasks such as antivirus scan, hard disk defrag, or to create an index that requires a lot of memories to operate. This method is very useful because some applications can only run when the computer is idle.
