Using your spotting scope in bright lighting conditions may result in a view that has glare or flares from sources of light in the direction you’re looking.
A lens shade acts as a tube extension of several inches to do just that: provide shade from the distracting light that’s ruining your view. Most scopes will have at least an inch-sized length of the main tube extending beyond the lens. Some larger-aperture spotting scopes will have fixed lens shades that are much longer. Retractable shades save space; they can be found on Celestron's Regal M2 ED spotting scopes. Just pull these gently forwards to shade your scope.
Under conditions of cool temperatures or cool optics and high humidity, any lens shade will also prevent dewing on the lens of your scope. In this case, your lens shade acts as a dew shield or dew cap. The longer your dew shield is, the longer your lens will remain unfogged.