Most Helpful Girl
It's a amalgamation of traditional etiquette and security.
Etiquette rules span across a lot of polite society, and just don't always trickle down to most of the middle and working classes- things like the correct things to say to each the bride and the groom at a wedding (it can be an insult to the bride to say congratulations for example), cutlery outside in, - it's just a lot of this doesn't particularly matter to those lower down the social ranks, or there are rules for situations we just don't end up in. It maintains dignity, and propriety.
Do you remember when a french tabloid published pictures of duchess Kate topless? they try to protect them, and ensure they aren't objectified, and are criticised as little as possible.
The autographs aren't just about forgery, but also to do with spreading time fairly on visits, and avoiding things being resold unfairly.
The act as figureheads, draw in tourism and lead hundreds if not thousands of humanitarian and charitable efforts- yes, they live pretty lavishly and the wealth isn't fair especially compared with those who have nothing- but they live pretty full and tiring lives ensuring their efforts are worthy (William has only recently stopped piloting air ambulances).
Most Helpful Guy
That's what makes them royal. Etiquette seperates the nobles from the peasants. Think of it as a secret handshake with your best friend. It creates exclusivity.