Lumens is a specific form of measuring light output commonly used in the front projection industry to determine how much light one needs for a specific size and type of screen in a specific application for proper contrast ratio. To much light and you will not get your blacks black enough making dark scenes washed out. Not enough light and the image will lose it's punch and vitality appearing dim. Either scenario reduces the contrast ratio which means imaging that looks dead or lifeless.
This specification has little value to consumers or professionals in common consumer applications as the screen sizes, 85-110" diagonal 16:9, and the projectors used are fairly common for those applications. It is used primarily in custom imaging applications where a professional is designing a custom environment using abnormally large screen sizes, greater than 100" with professional or multiple projectors or in digital cinema applications.