That's inaccurate. The ability to run unsigned OSes while not affecting the safety of a signed OS is useful for beta versions of macOS past their due date, and most importantly, older versions of macOS that Apple no longer validate themselves.
Apple refuses to sign any OS they deem not up-to-date on iOS. I presume the same thing is coming to macOS soon, with the out that you will be able to install those older versions in the other OS code path.