Servers Split Up?

My newest client has a network set up with a domain controller,
a web server, a mail server, and a computer hooked up to a
back-up raid all on 4 different machines running SBS 2000. They
have Novell, but don’t know what it is running on. All the
workstations are 98 and they have a custom app that runs on
DOS for purchasing etc. In my experience one server has
handled all of the above tasks for a small office. Is there a
reason you would want to separate it? With 2 NIC cards you
could achieve the same thing on one machine right.
Thanks for any thoughts,
Mike