Pooling does several things.
One, by having one bean per instance, you’re guaranteed to be threads safe (Servlets, for example, are not thread safe).
Two, you reduce any potential startup time that a bean might have. While Session Beans are “stateless”, they only need to be stateless with regards to the client. For example, in EJB, you can inject several server resources in to a Session Bean. That state is private to the bean, but there’s no reason you can’t keep it from invocation to invocation. So, by pooling beans you reduce these lookups to only happening when the bean is created.
Three, you can use bean pool as a means to throttle traffic. If you only have 10 Beans in a pool, you’re only going to get at most 10 requests working simultaneously, the rest will be queued up.