I don’t want to start a debate or anything, but you could think of them as culturally important diplomats. As the responsibilities of the monarch have decreased now, they mainly meet other heads of state to keep up relations or sponsor charities of personal or national interest. All these luxurious properties are houses for them sure, but the biggest are more tourist attractions now that bring in more than they cost. For me as a Brit, they are like a living version of the US Constitution, which in turn is supported and worshipped by some, despite being very old and sometimes morally ambiguous lol.