This move should tell Prince Charles and Prince William that Her Majesty, The Queen wants Prince Andrew's company and support at an important function for her. They both get something out of it. This sort of thing isn't unusual with a frail elderly person who relies on one child more than the others for companionship and a trusty person to assist. Clearly, neither Prince Charles nor Prince William are in a position to be her chauffeur/ assistant and perhaps Prince Edward is just not as close to his mother.