Yea, my understanding is that the split between / and /usr used to be more or less: the drive they used for / ran out of space, so they mounted another drive as /usr. As a consequence, / became where you put stuff that was essential during early boot, while /usr was where you put everything else.

But these days, "early boot" is handled by initramfs and we've all got drives large enough to not need the split anyway.

It was also useful for many machines to share a single /usr over NFS