It's amazing how early design mistakes of successful software end up getting after-the-fact justifications that are ardently defended by its fans.
Example: /usr was the disk for users data in the original Unix (old docs use /usr/ken in examples after K. Thompson). When the root disk got full they started storing programs there and /usr/bin was born.
People now give backronyms for "USR" and justify the /usr though the rigid "partitions" model is flawed (and was even abandoned in Plan 9).