Part of what makes the setting so important in Dear Mr. Henshaw is that the reader doesn't know exactly when or where the story takes place. While it seems that the story occurs in the 1980s, the issues and topics broached can occur to any child at any age and in any city. This allows the reader to put themselves into the position of Leigh Botts.
It is also important to note the age of Leigh, our letter writing protagonist. As he navigates the issues many young, adolescents go through, we see him grow starting with the first letter in 2nd grade going through his 6th grade letter. He must grow up and understand serious topics like divorce, an absent father, a working mother, and moving as well as more age appropriate issues like lunch thieves, missing dogs, an absence of friends, and (most troubling to a young boy) a broken tv.