I don't get the relevance?
France has points of interest that attract tourists, so does the UK. One of our attractions is the Royal Family.
The Royal Family costs each person 65p per year, which I think is pretty good value. Even if tourists don't spend money directly on the Royal Family they do create interest in London/UK. The Royal Family alone won't attract tourists to fill our hotels, bars and restaurants but neither will Madame Tussauds. We need a collection of attractions to bring people in.
The Royal Family is cheap to keep. I like the tradition of it, from the Palace itself to the silly uniforms the guards wear.
arkitect's link explains much more clearly than I could.