Ultimately, the Royal Family is a hierarchical system, and only one person may be at the top of it. Right now, that's the Queen. One day it will be Prince Charles, and after him it will be William. When those events occur, Camilla and Catherine will face the same problem that Prince Phillip currently has to deal with; even in marriage, they're not equal to the Head of State.
Prince Phillip has to walk two steps behind the Queen at all times, to show deference to her rank. If she stops, he also has to stop two steps behind her. Sometimes, this results in awkward situations, as we see in this picture. Unable to step forward after getting out of his seat, Phillip must remain face to face with a curtain until his wife chooses to either move or sit down again.