Holes.

a patrol car pulled alongside him. a policeman asked him why he was running. then he took the shoes and made a call on his radio. shortly thereafter, stanley was arrested.
it turned out the sneakers had been stolen from a display at the homeless shelter. that evening rich people were going to come to the shelter and pay a hundred dollars to eat the food that the poor people ate every day for free. clyde livingston, who had once lived at the shelter when he was younger, was going to speak and sign autographs. his shoes would be auctioned, and it was expected that they would sell for over five thousand dollars. all the money would go to the homeless.

why does sachar include this description of the shoes in the flashback?

to show where the shoes came from
to show how valuable the shoes were
to reveal why stanley was arrested
to give background on the homeless shelter