He reveals his location to highlight the injustice of the situation.
Explanation:
By putting the header information in bold, King highlights that he is Martin Luther King jr. and that he is being held in Birmingham City Jail for peacefully protesting for his rights. With that, he shows that he is imprisoned for an unjust and racist cause.
As you may already know, the first amendment to the constitution allows protests to take place peacefully without the protesters suffering any kind of punishment. So the fact that King evidenced that he was in prison served to show how illegal, racist, unjust and wrong this prison was.
