In the original Unix /usr housed homedirs for user accounts as well as other stuff of general interest to users.. this remains partially true today.
You can find a note from Dennis Ritchie's, as seen here
In particular, in our own version of the system, there is a directory "/usr" which contains all user's directories, and which is stored on a relatively large, but slow moving head disk, while the othe files are on the fast but small fixed-head disk.
So system stuff was traditionally kept in /bin and possibly /sbin, while "shared" user stuff may have been in /usr/bin instead.
I have a few older commercial Unix flavours for x86, they also typically put user stuff in /usr.
Having pages like hier(7)
are really helpful, all systems should document their preferred directory structure.