Definitely to save ones eyes from becoming sore. It might not be so bad when you're a part-timer or hobbyist, but after a few years of coding daily the brightness can take a toll on your eyes. Years ago I used to have to sit in the dark listening to podcasts/lectures/audiobooks to recover each day -- couldn't watch TV or go outside without dark sunglasses, my eyes had just become too sensitive to light. A dark IDE helps, as does taking regular breaks and making sure to turn down the brightness on your screen to the minimum necessary.