How much do events like Trooping of the Colour bring into the economy? The spending from the visitors/spectators must be huge. Most of those spectators wouldn't be wondering around London that day if it wasn't Trooping the Colour! That's spending on travel, hotels, meals, souvenirs, and other attractions.
You wouldn't get those huge crowds otherwise. They're only there to see the Royals. Yes, you may get some crowds if similar precessions happened without the Royals in attendance, but nowhere near the same numbers.
Same with royal "walkabouts" and official visits in other towns and cities all over the country. The places are heaving with people, all spending money in local shops etc, some staying in local hotels for short breaks etc. When Charlie Boy came to our town a year or two ago, one of my clients, a little Guest House got fully booked literally the day the royal visit was announced - that was out of season and he was otherwise empty and devoid of bookings that week!
Don't underestimate the pulling power of huge numbers of people who actively want to go and see the Royal, or even just the Royals passing in their car! Despite what people think, they're still very popular and they do drive economic spending.