Place holder root domain advantage

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I've been struggling with a domain design to choose. I've always read that
it is best practice design to create an empty place holder root domain to
hold the enterprise admin group and to hold the forest schema operations
role. Then have another domain to hold all users/groups/computers. The
alternative being one domain, that holds all of the above.

There is obviously additional hardware costs associated with the empty place
holder domain, but there isn't going to be much administrative overhead
since the domain is going to me basically unused.

What are the underlying reasons why the place holder root domain is setup
and should this domain design be favored in a large enterprise organization
vs the single domain model?

Thank you.


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