How is this for an explanation?
The main purpose of IIS is to respond to HTTP get page requests. Now think of all the applications that rely on web components.
WWW Web service for displaying web pages
SQL 2000. HTTP queries and also the Monitor service
Exchange uses the SMTP virtual server
OWA (Outlook Web Access) thin clients of Exchange
FTP if you allow client to load up files
NNTP another virtual server this time for network news groups. NNTP is also needed by Exchange 2000/3
SharePoint Portal server needs the FrontPage Web Extensions
Web or Internet based printing another use of Web Extensions
Taken off this site http://www.computerperformance.co.uk/w2k3/W2K3_IIS6.htm
Here are some other links that might help you out as well.
Microsoft Windows Server 2003 Internet Information Services (IIS)
A webcast on What's New in Internet Information Services 7.0 for IT Professionals
A webcast on Internet Information Services 7.0 Diagnostics and Troubleshooting
Internet Information Services (IIS) 7.0
A virtual lab on IIS 6.0 Application Pools
A virtual lab on IIS 6.0 Compression
A virtual lab on IIS 6.0 HTTP Caching
Now that's how you explain things to people!!!!!