if you have only one
servlet then... yes it's possible to skip it, but if you want to build a complex application, web.xml is quite necessary.
you can:
a) define your servlets there (name, calsses, location, start-up control);
b) put some JNDI values there;
c) define resources;
d) define security roles;
e) configure sessions.
+ much more
more details at:
http://kb.atlassian.com/content/orion/docs/deployment/web.xml.html and in many other places.
hope that helps a bit.