It exists because, long ago in a galaxy far far away, a sysadmin ran out of space on a drive. The system was split between two 10MB(?) drives, one was / and one was mounted at /usr, for User data. They moved some of the programs to a folder for a dummy user, /usr/bin, and put that in everybody’s PATH.
Everybody kept on doing things that way ever since. Social momentum is funny that way.