Why is dark adaptation a requirement for effective astronomical observation?

It maximizes the sensitivity of the retina to low light.

Dark adaptation is a physiological process where the eyes spend a significant amount of time in total darkness to reach maximum sensitivity. During this period, the rhodopsin levels in the rod cells increase, allowing the eye to detect the faintest possible inputs from the night sky. Exposure to even a small amount of white light, such as looking at a smartphone screen, triggers the iris to constrict and resets the chemical sensitivity of the rods, effectively ruining the observer's night vision. Proper dark adaptation requires at least 30 minutes of undisturbed darkness to ensure the eyes are capable of perceiving the faint, subtle light of deep-space objects.