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RDKirk has the right idea.
Any stray light on the monitor will reduce the visible contrast of images displayed on the monitor by filling in the blackest blacks. The brighter the light the more the reduction in contrast. Some of the light will be coming directly from the source and the rest is reflected off walls, ceiling, etc.
In a general purpose room where the lighting cannot be altered much without considerable expense and/or interfering with other activities in the room, a monitor hood is a good thing - provided that it is dark enough so as not to reflect stray light onto the screen, and also that it protrudes far enough at the top and sides to be an effective shield against direct light sources. That is easy if the light is pretty much above you but quite difficult if the light is pretty much behind you.
Hoods that protrude just a few inches are close to useless. 12 inches or more is likely to be somewhat beneficial. If they don't protrude at the sides as well as the top of the monitor then they will offer no protection from windows or lamps beside the monitor.