Why did Prince William and Philip have to wear the uniform of the royal household calvery on the queens birthday?

Prince William and Prince Philip wore the uniform of the royal household cavalry on the Queen's birthday to honor her and participate in the ceremonial celebrations. This tradition reflects their roles within the royal family and the military, highlighting their connection to the British armed forces. The uniform signifies respect and allegiance to the Queen as the head of state, particularly during significant occasions like her birthday. Additionally, it showcases the enduring military heritage of the monarchy.