Active Directory holds for itself the root of its domain. So if you use the same domain for both your corporate Active Directory and a website then you suddenly have a conflict.

This mostly isn't an issue because Active Directory is usually only used on internal, non-routable DNS spaces. So in the public DNS space a company can default forward all requests for the root domain over to the corporate website and everything works. Internally, requests for the root domain will route to your nearest Active Directory server.

Of course best practice design is to use a different DNS name space for public and private zones, but most companies (especially the small ones!) don't bother.

So anytime the root name space is dedicated to any non-www service then some other name is used to route traffic to the website. For years the default alternate name has been www. You could also use the name of the specific web server, but that's harder to remember.

Agreed. It's a powerful perspective.