It's an emergency light. If there's a power cut/fire this light will come on so you can see. It's battery powered and the green light means it's working.
It's a night light, so if you go to the bathroom in the dark you don't miss.
The human eye is the most sensitive to green light, so the least amount of light is needed for you to be able to see things. Green lights are excellent for night lights, due to this reason. You don't lose your night vision, and you see more detail.
Humans have best retina stimulation in blue light, not green light.
The real reason I suspect the light happens to be green is that green phosphor is relatively inexpensive.
Blue light could be disruptive to circadian rhythm while green light is somewhat less so, but I guarantee this was not part of the calculus here. It is just being thrifty. Circadian rhythm benefits are just a happy accident.
When fully light-adapted, the human eye features a wavelength response from around 400 to 700 nanometers, with a peak sensitivity at 555 nanometers (in the green region of the visible light spectrum). The dark-adapted eye responds to a lower range of wavelengths between 380 and 650 nanometers, with the peak occurring at 507 nanometers.source
Wut? Blue? Are you sure about that? Afaik the peak is around 555nm, yellow green. Why do you think we have "high visibility yellow" vests and not "high visibility blue?"
IIRC 555nm (or whereabouts) stimulates the L and M cones simultaneously.
Emergency light, so you can see when doing your business in the middle of the night. Fun fact, a large number of accidental deaths happen in the toilet.