One of an illegal's worst fates is being deported. Since the law requires a lawful stop as a precondition for immigration checking, the best way for an illegal to protect himself is not to engage in the illegal conduct that would provoke such a stop. Hence, we see one of the best and most immediate effects of the law. This is an excellent reason for every state to have such a law.
Yes officer, I'm pretty sure that person was jay-walking.