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.