What is the reason for the large minimum brightness of MacBook Air (or other devices)? When I use my MacBook Air in darkness for various reasons (e.g. causal stargazing or working late in close quarters) the last brightness notch before "black" is way too bright for me. 

There are add-on work-arounds (see for example Macbook air screen is too bright even on the lowest setting) so I'm not asking how to get lower brightness.

I'm wondering if there are some hardware issues when operating at very low brightness that should be avoided, or if it's just a question of difficulty to assure the customer display quality at very low brightness, or something else?
Have you tried to split the last notch in 4 part, pressing Alt+Shift+F1 or Alt+Shift+F2 ?

It allow you to have other three level before black screen. It will works also with audio.