DAO / repository: Good practice return value after insert / update

Although probably a trivial question, I've always wondered about this.

Usually, after insertion into the db, it seems common practice to return the id of the business entity.

@Override
public Long createUser(UserEntity user) {
    em.merge(user); 
    em.flush(); 

    return user.getId(); 
}

Is there a convincing reason for returning the id instead of the business object reference itself?

Similarly, I've seen update return void, while it might just as well be an id / User.

If I were to write a DAO / Repository for others to use, what would be the advised return value (if any), and why?