Firstly it gives us time and place. We see the Christmas lights- like netted starfish, sagging over Regent Street. And we notice how isolated the royal couple are- travelling inside their modern version of a glass coach. They're warm in there- wearing evening dress, not coats. What has been captured is the exact moment when the glass wall is broken- and the air of the street whistles in.
Oddly enough we don't see the assailants.
But there are two other people in the frame. They're out in the cold and dressed for it. One is a photographer. There is always a photographer. This guy is capturing the image from the rear. He'll have got a good shot- and he looks gleeful about it- but not as good as the one we're looking at.
Then there's the chap on the right, hooded and bundled up like a Breughel peasant. He might be a demonstrator (he's the right age) or he might just be a passer-by. Anyway he's Everyman and he has concerns of his own. As Auden pointed out in his famous Breughel poem, incidents that will be remembered for always take place "While someone else is eating or opening a window or just walking dully along". Our man, our Breughel peasant, looks vaguely bothered, but he's not looking at what we're looking at, he may even be unaware of it. Instead he's looking a little to the right, with a long-distance gaze, at something behind our backs.