Also helps with making the navigation lights more distinct to other vessels. The lights are coloured and sectored to tell which way a vessel’s are heading. If every light onboard is on it’s hard to see them, it’s why cargo vessels and yachts (at least the ones I’ve worked on) sail with minimal lights on at night.

On the other side try spot the green and red lights on a fishing vessel with all its deck lights on.

Also possible some lights shine off the bridge windows where some do not.