There are 3 kinds of monitor, CRT, Plasma and LCD. CRT and Plasma have flying electrons which impinge on phosphors on the inside which causes the phosphor to glow. In any CRT, there are three dots per pixel, red, green and blue. By selectively lighting the dots, they make colors. The problem is the electrons impinging on the phosphor in exactly the same place over time will cause the phosphor to eventually burn, cause a spot dimmer than the rest. Eventually, with a fixed image there for a long time, when you turn off the CRT or Plasma, you see the burned spots on the inside surface. So, with the CRT, which is where this all started, the screen saver was simply a moving image. As long as the image was moving the burn of the phosphors was equalized all over the screen rather than in one place. I have an old Novell Network Server and the monochrome green CRT it has shows clearly the menu that was burned in because no one turned off the monitor and there is no screen saver in the server console program. It ran for literally years with virtually the same menu showing on the screen. Just because it shows burning, does not mean it does not work or is unreadable. It is clearly legible when it is running. An LCD is immune because the colored pixels are made by darkening crystals in the LCD display and a cold light is used to light the screen. Since no electrons are landing on phosphors, no burning iin the sense of a CRT or Plasma can happen.With an LCD, you have a different kind of problem, the cold lamp lifetime. If left on all of the time, some model have a shorter lifetime in days on the calendar than ones turned on and off. However, if you look at total hours the lamp is ON rather than calendar days, turning it on and off lessens the number of hours lit by as much as 25% but it runs for many more days on the calendar.The lamp in a LCD set is the most likely point of failure.