A couple of bullet points:
From version 4, Liferay requires only web application server framework (servlet engine) to run on, which in this case is represented by tomcat and jetty.
Web application server is a very little piece of J2EE, and by definition it is supposed to be used as a presentation tier only, meaning:
- processing of input data (display of form, form validation, etc) and passing it to the backend for further processing, and
- rendering of data that comes from the backend.
This is all Java Portlet Specification is about, not much else. And companies have many channels of data input/output, not just a web browser, portal is one of those.
The backend system usually does the heavy stuff. For this purpose you would really need a fully equipped J2EE application server.
You would not want customer relations officer to be a brain of your business, don't you?, but this is exactly what happens when your web application server contains entire business functionality.
While such approach is perfect for small/social sites, that is a dangerous and costly direction for growing and larger businesses. That's why you would need a J2EE packed portal bundle. Still, such bundles are good for smaller enterprises only, for real-time processing systems, medium companies and up you would need to plan a physical separation between processing tiers, i.e. for portal to be lined up for communication with remote J2EE app server(s) or farms.
[my personal opinion only, customers are allowed to disagree]
What exactly is the benefit of combining JBOSS with tomcat? Wouldnt JBOSS on its own not do the job?
Jboss has many bundles, all of them rely on 3rd party web app servers, liferay has picked Jboss bundle with tomcat.
Same story for the rest of J2EE servers. Usually they do not have own web app server, no need.
