IIS necessity on forest root DC

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From: cris (anonymous_at_discussions.microsoft.com)
Date: 03/16/04 

Date: Tue, 16 Mar 2004 09:16:08 -0800

We have the following new AD structure:
   DC1-EMPTYROOT --> empty root DC for forest (domain=FORESTROOT.ad)
   DC-ALPHA --> DC for "new tree in existing forest" (domain=DOMAIN1.company.com)
   DC-BETA --> DC for "new tree in existing forest" for (domain=DOMAIN2.company.com)

While in the process of migrating to Exchange 2003, we were told that in order to complete ForestPrep and DomainPrep, we have to install IIS on the root DC (DC1-EMPTYROOT). Why would this be the case instead of requiring it just on the Exchange server itself?