The short answer to this is that you normally use the Multiserver without a Web server during development and with a Web server during deployment. A Web server at its core is just a program that returns files. The Multiserver contains a servlet that gets and retrieves static files. This program is often fine for development where performance is unimportant. However, at deployment time you will want to use a Web server to serve static content and the Multiserver to run servlets generating dynamic content.
Web servers are better at serving static content because they optimized to do that.
They have sophisticated caching schemes and are written in native code. The servlet runner excels at running servlets, but can serve static files as a convenience to consultants developing on a laptop during a 5 hour plane ride. The Enhydra Multiserver is servlet 2.2 compliant. Complete instructions can be found here. Complete instructions can be found here.