One clear advantage of monarchs that I can see, are that they have an incentive to grow and expand their tax base. That typically means long-term planning (but doesn't ensure it, which is a disadvantage the UK parliamentary system seems to mitigate).
Hereditary absolute monarchy is the same thing, but for selecting heads of state. Who's in charge? The guy with the biggest army. What powers does he have? All of them. Who succeeds him when he dies? His firstborn. It's dead simple to implement, which made it an attractive solution in times before any semblance of mass communication. But in practice it means the quality of your head of state is totally detached from their actual talent at serving as head of state: the first guy in line was just good at leading an army, and the rest of his descendants are just randos who won the birth lottery. It's not a good solution unless you're willing to make loads of sacrifices in order to have the simplest system possible.
(And yes, of course, the UK is not currently an absolute monarchy, but you appeared to be asking in a general sense.)
Sure, now that the top position is entirely symbolic in the vast majority of monarchies, nobody fights over it. But the if the UK Monarch was in any sense "in charge" of anything, we'd surely see the US propping up Harry and Meghan as the true legitimate heirs and we might even let them borrow a couple carriers to "persuade" Parliament of the fact. Or whatever.
They are given a lot of money.
They can (and regularly do) veto legislation if it would harm their (vast) business empire.
They are not elected.
The burden of proof is on you to say why this is a good institution.