Seam proxies the EntityManager or Session object whenever you use a Seam-managed persistence context or inject a container managed persistence context using @PersistenceContext. This lets you use EL expressions in your query strings, safely and efficiently. For example, this:

User user = em.createQuery("from User where username=#{user.username}")
         .getSingleResult();

is equivalent to:

User user = em.createQuery("from User where username=:username")
         .setParameter("username", user.getUsername())
         .getSingleResult();

Of course, you should never, ever write it like this:

User user = em.createQuery("from User where username=" + user.getUsername()) //BAD!
         .getSingleResult();

(It is inefficient and vulnerable to SQL injection attacks.)