How does the UK benefit from the royal family allowing this, rather than just having a non-monarchist government that does the same thing?
Also, they provide a lot of face-time for the government. They show up at a lot of events, the kind of thing for which you might hire a celebrity to give it a sense of occasion. The actual dollar value of that is hard to estimate, and it's less valuable than the tourism, but the fact that people still want it suggests that it's providing something of value.
Thats the point, its not trivial to sell it off, because it was set up to counteract a fooling king in exchange for a stipend.
This system was explicitly designed to take an asset from the crown and place it in trust so that the king could get a steady income, ostensibly from assets that had unpredictable income.
Thus the crown estate was born, which has a bunch of rules, most notably about taking on debt.
(dartmoor is owned partly by the crown)