Well Amila, when your application is deployed on a distributed environment, the session information will transfer remotely to other JVMs in the same environment. In this case, session information should have to be serialized and deserialized in order to pass it as a remote object. Hence
you should care about HttpSessionActivationListener an must implement it's sessionDidActive and sessionWillPassivate methods appropriately if it is needed. It is required to do so, if the session stores some connections pooling data / or resources which must be committed and/or closed before serializing them. For the SCWCD exam you are NOT expected to have a deep knowledge about distributed environments. Instead of that, basic understanding about HttpSessionActivationListener is sufficient for the exam. And keep in mind that multi-tier (n-tier) architecture is completely differ than distributed environments.
Devaka.