Beans that reside outside a transaction scope normally provide some kind of stateless service that does not directly manipulate data in a data store. While these types of enterprise beans may be necessary as utilities during a transaction, they do not need to meet the stringent ACID requirements of a transaction.
Consider a nontransactional stateless session bean, the Quote EJB,
that provides live stock quotes. This EJB may respond to a request
from a transactional EJB involved in a stock purchase transaction.
The success or failure of the stock purchase, as a transaction, will
not impact the state or operations of the Quote EJB, so it does not
need to be part of the transaction. Beans that are involved in
transactions are subjected to the isolated ACID property, which means
that their services cannot be shared during the
life of the transaction. Making an enterprise bean transactional can
be an expensive runtime activity. Declaring an EJB to be
leaves it out of the transaction scope, which may improve the
performance and availability of that service.