You detect light to know when to wake up or something is happening. Totally blind people also have the mechanism and why they know when it is morning (unless they are missing the organ). I am sure there are other reasons.

I’m now imagining falling asleep around a campfire, and there’s… a shadow

My guess, we evolved in hot areas so would often rest during the hottest part of the day. When you’re napping your brain can probably notice if a shadow blocks the light alerting you to a potential threat.

That’s totally an uneducated guess though.

Probably so you wake up when it’s light out