It’s not a requirement under any federal aviation regulation (FAR). It’s a safety precaution companies take to help speed up the process during an evac. It can also be useful for safety crews to see inside. E.g Emergency crews can make sure a section is clear of people before they cut through metal. Lastly, it’s smart to look out the windows and check for hazards prior to you jumping out on a wing. Visual check for spinning engines, fires and sharp edges that might slice you open.

To provide further I formation; When we do evac certifications, we have to black out every single window. The hangar we used is blocked from all light. We get RANDOM (has to be random) people off the street and stick them in the plane. They get a safety brief just like a typical passenger. Then they have to prove the plane can be evacuated in under 90 seconds.