I am using Struts2 - Spring - Hibernate on standalone tomcat server. However, I heard that someone is using Jboss while also running Struts2 - Spring - Hibernate web appli (without using EJB). Why Jboss is needed in such a case? Is there any benefits for using Jboss rather than standalone tomcat?
Tomcat offers clustering, JBoss does too. Whether it is more effective depends on your application's needs. Since you're asking, I think Tomcat's clustering will do nicely for you. :-)
If you write your application according to the JEE spec and use Tomcat without relying on any of its Tomcat-specific features, you can move to JBoss (and endure JBoss start times) at any time in the future.