I believe that this property allows to instrument not only entity classes but other code as well in order to intercept field access.

Consider following code:

@Entity public class MyEntityClass {
    @Id public long id;
    @Basic public String data;
}

In case of extended instrumentation it will transform this method:

@PersistenceContext em;

void businessMethod() {
    MyEntityClass mec = em.find(1l);
    System.out.println(mec.data);
}

To something like this:
In case of extended instrumentation it will transform this method:

@PersistenceContext em;

void businessMethod() {
    MyEntityClass mec = em.find(1l);
    System.out.println(mec.$javassist_read_data());
}

This will allow lazy loading work even for public field access strategies