Re: With two-faced "friends" like Dyson, Britain doesn't need enemies
It isn't actually a national treasure role. Head of state represents the nation as a whole. While it's desirable that the various branches of the state, military, judicial, civil or whatever should be responsible to the nation humans really work best if their relationship is with another human rather than an abstract entity. That's what having a head of state provides. OTOH separating head of state from head of government avoids gathering all the power into one pair of hands which is generally a Good Thing.
You can debate at length how best to achieve that. At least a hereditary monarchy avoids making it a political office which is hard to avoid with an elected head. It also means that you don't have someone whose event horizon is no more distant than the next election. It's not a fate I'd wish on anyone, however - I'm slightly older than the king and a long time retired whereas he's only just got started. It also means that you have to take pot luck to some extent although we do seem to have solved that problem - it took a lot less time to get rid of Edward VIII than it did to get rid of BoJo.