Why is it necessary to perform manual clean-up of APFS partition/container when removing one copy of macOS on a dual macOS install?

In https://apple.stackexchange.com/questions/334088/remove-macos-from-a-apfs-container/ there is an answer by @"David Anderson" that states that if you install 2 copies of macOS on separate volumes within the same partition/container, then try to remove one, you need to perform this multi-step clean-up. My question is why is it necessary to perform this clean-up? How do these commands know not to remove the recovery for High Sierra main or the second copy you installed? And if you remove these partitions, is there still a Pre-Boot and Recovery partition that remains? Please clarify.