Why to put the Web server in front of Tomcat at all?

http://people.apache.org/~mturk/docs/article/ftwai.html#cooliris

A. First thing to ease the load from the Tomcat is to use the Web server for serving static content like images, etc.

B. Second thing is creating an DMZ (demilitarized zone). Putting Web server on a computer host inserted as a "neutral zone" between a company's private network and the internet or some other outside public network gives the applications hosted on Tomcat capability to access company private data, while securing the access to other private resources.

C. The need for the custom authentication for example.