It’s nice to see that these guys switch it up depending on who needs that supportive hand. In this picture, Kate is pregnant, and William is making sure that his wife has his support. Although we know Kate can handle herself, that reassuring touch can be pretty nice.
They’re clearly out for a very formal event, as the guard is behind them, and William is dressed in his official uniform. Pregnant or not, they have to attend, and they can’t be grinning and smooching, but they can offer a hand.