My attempt: King uses imagery of actual experiences of Black people in America to explain why they have been waiting long enough. If you were an outside observer, it might make sense that civil rights advocates should wait a little longer. It can't be that bad, right? The letter helps put you in the footsteps of a 6-year-old girl who can't go to an amusement park because only white people are allowed, and it shows why it is hard for people to wait for change. Without the imagery, the letter would not be nearly as effective; you would not be able to see it from the perspective of a Black person.
