Sorry if someone already posted this, but this is a direct quiote from How to Read 13. Takeshi Obata's production notes about L:
My editor mentioned that I should make his face capable of looking cool based on the angle. To me, bags under the eyes can be cool... For example, the main character in Devil Man, Akira Fudou, has bags, and I thought they really looked cool. Also, one concept was that he had "dead eyes," so I gave him mostly no eyebrows and all-black eyes. Black eyes usually make a character look goofier, but the bags help sharpen the character's gaze. You get a feeling of mystery, or that you can't tell what the character is thinking, and I really liked that. Also, the bags under his eyes bring out a lot of speculation about his lifestyle and his past; I later realized just how useful they are. In Obha-sensei's thumbnails, L didn't have bags: he was a very plain face with no expression [laughs]. It was great and I wish I could have used it as is.