Or you're implementing an odd algorithm where a pointer to the paper, etc, describing it is located so that co-workers, etc, know what's going on. Or you're working around architectural limitations, such as doing work on microcontrollers, where you don't always have your preferred integer size to work with. Or working around bugs and/or poor naming conventions in other peoples' software. Yes, your code may be beautiful and cause angels to sing, but it's going to be working with code belched from the deep bowels of hell, and giving others clues as to why you're doing things in non-intuitive manners is not just useful, but needed.