Best answer; read KS EJB 2x;
that would help to let you know containers role & importance; & EJB life cycle.
but for now, container (a box) in which EJB resides, provides transaction mgmt, security, bean pooling, ..... etc.
In other case, if you keep your business logic in (POJO)
Java class, you would need to take care of all services which are provides by container.
hope this would help.