That only moves the question back a step. Why have a bin user that owns all the userland binaries and keeps them directly under its own home directory? What does that accomplish? Something to do with per-user quotas? Or making `ln /usr/bob/a.out /usr/bin/newcmd` cheap?

I note that there's also a bin group. Was /usr/bin mode 1775, and anyone who should be allowed to install binaries (and update/remove their own installed binaries) was made a member of the bin group?

Also, I note that there's no sbin user. (There is, however, a sys user. But only on Linux, not on BSD...)

sbin is a SYSVR4'ism ... it's complicated. /etc had a bunch of binaries in it like fsck. MACH systems had that as well (well technically it was linked to /private/etc which then linked the binaries to /usr/etc - here's an openstep screenshot I just took: http://9ol.es/usr-etc.png). It's /etc/mkfs /etc/route ... they got moved to /usr/sbin/ and /sbin in R4 - by then, SYSV systems were using /home and the initial purity of every owner of every process being an equivalent class user had been broken by pragmatism.

The invention of /sbin/ /usr/sbin is all R4, see https://archive.org/details/unixsystemvrelea0000unse_t8i3/pa...

Unix 1980s history is a complicated mess ... that's why there were multiple conflicting standards groups that tried to fix it. The X/Open group had the "single unix specification" which is different from POSIX and say, the Open software foundation (which did OSF/1). Here's a nice crash course: https://en.wikipedia.org/wiki/Single_UNIX_Specification

Explaining the history with the motivations is probably around 10,000 words. AFAIK it hasn't been written. I'd be one of the handful of wackos who'd actually do that and I haven't seen it.

It let them add a second hard drive after the first one filled up (5MB each, as I recall).

You misunderstand. My question was why they bothered to make a real "bin" user to have /usr/bin as its homedir; when, if it had just been purely an issue of shoving some binaries over to the /usr drive, they could have just created a /usr/bin owned by root:root and leave it there; or, if they really cared, created a /usr/root for "a secondary root-user homedir that lives on the /usr drive" and then made a /usr/root/bin.

That I don’t know. My guess is that it came later, so that no new user could be called “bin” and thus inherit ownership of those files. There are some historical sources that might shed light on the matter, but someone would have to do some work.