Uhh, that is an extraordinary way to do. Nice find.
But there is no real use case. Jus to prevent beginners to go through it.
Baking from nice finished high res to low res with UV is recommended.
And it is much more predictable than painting black & white on low res and seeing the results after a serious procedure after.
Thank you for your hard work, anyway. There will be once a moment I remember this.
One problem could be, if you bake the normal map directly from a high res mesh, that you always need the FLAT low res mesh, things do not work properly with the decimated HiRes mesh. You should never forget to work over layers.