A 'Statefull Session Bean' is an EJB component. Many servlet apps are written without EJB. For those apps, the ServletSession is the most convenient place to store data across web hits.
In terms of a design consideration, Servlet's session is more relevant for maintaining a user's web session. Servlets is not best suited for the application specific data which could consist of data used for business logic. Stateful session beans are useful in this scenario.
Swamy
In the morning there is hope; in the afternoon, fulfillment; in the evening, memory; at night, peace.