You’re overlooking a very common reason that people setup a homelab - practice for their careers. Many colleges offer a more legitimate setup for the same purpose, and a similar design. But if you’re choosing to learn AD from a free/cheap book instead of a multi-thousand dollar course, you still need a lab to absorb the information and really understand it.

Granted, AD is of limited value to learn these days, but it’s still a backbone for countless other tools that are highly relevant.