Environment properties override allows to set individual settings of environment and atmospheric effects in each pass.
Also important is the ability to include only the necessary elements to render. For example, when you rendering pass of the main image, you can include all additional render elements you need (reflection, refraction, etc.), and when you render pass of additional mask turn off all render elements and save only beauty. This saves render time and hard disk space.
One pass may contain only one Environment properties override.