In the epilogue of Into the Wild, what is the significance of Walt and Billie visiting the bus in the Stampede Trail?

In the epilogue of Into the Wild, Krakauer describes traveling with Chris's parents to the site of the bus where he died. Billie and Walt have been devastated by their son's death, but they are both glad to see where he lived and died. They take in small reminders of his presence there and leave a plaque in his memory.