Comment by nyrikki

While practically useless in reality /usr/local is `site-local software`, E.G. software that if you nfs mounted /usr, would be local to the `site` not the machine.

The BSD ports explanation is a bit revisionist I hate to say, this all predates ports.

It was a location in a second stage mount you knew the upstream wouldn’t overwrite with tar or cpio. Later ports used it to avoid the same conflict.