Security

Screensaver software can to some extent perform  a security function.  For example, a lot of screensavers  have the option of asking  a user for a password  to permit him to start  working. However, there is a way to enter just by restarting the computer. But it works if the owner of computer set an option to log in automatically  upon start-up. 

Purpose

Before LCD screens appeared, the main technology of computer screen production was based on cathode ray tubes (CRTs). The disadvantage of  CRT screens  is appearance of a dark shadow which happens due to the changes of phosphor coating after the same image is displayed for a long time. In fact, the devices with CRTs  have a problem with phosphor burn-in as well as plasma displays.