The detached criteria allows you to create the query without Session. Then you can execute the search in an arbitrary session.
In fact you should think carefully when using a detached criteria using another, or a new, session (no cache, and creation of the session).
They are most useful for join conditions, subselects, and to query outside the current session.
Another common use is for code reuse. Many developers declare them as static queries and execute them using the underlying session from different DAO.