The concept of average reflectance is extremely important in photography. It helps in keeping an accurate white balance check for your images. Though in a technical sense the way modern DSLRs (and for that matter mirrorless camera systems) meter, it is still a flawed system. These cameras use the reflected light system for metering a scene.
Thus, brighter the color, brighter is the metering of the scene. Thus, a predominately snowy scene will be metered very bright. But the sensors are designed to ‘average’ out the scenes. Thus, when a bright scene is confronted with the camera will try to underexpose (compensate) and bring everything down to middle gray and in the process darken the scene.
On the other hand, when darker colors dominate a scene, the scene will be metered darker than they actually are. The result will be overcompensation.
An 18% gray card will help prevent both as it keeps a neutral reference and a one-touch solution to correct this problem.