Meh, as far as kings go, he was alright. I've also met the new king (Håkon), and he's pretty decent too.

Harold was a good one though

The Nordics and some other countries in Europe uphold a powerless monarchy.

... Essentially they're a kind of celebrity...

They’re called kings but they don’t even make insane pronouncements or make nonstop executive orders demanding that the tide will not come in.

That's a weird take to have while a monarchy is forming in full view of the whole world in real time.

Of 30 countries in the EEA, 8 are monarchies: BE, DK, ES, LI, LU, NL, NO, SE.

EDIT: If I counted correctly, Europe as a whole has 4 more monarchies than that: AD, MC, UK, VA

If your question is genuine: Kings in European constitutional monarchies (or at least in Belgium) play a political part outside of the political circus of constant posturing.

First, they help to broker deals between parties elected by the people. In Belgium we have the awesome proportional representation, meaning that we escape the drawbacks of first-past-the-post. But we have a shitton of parties that need to negociate and compromise. The king is the showrunner but only elected officials are in power in the end.

Second, they perform diplomatic missions. These missions rely on personal bonds maintained over decades are more stable than for a president that can change every few years. The phone calls of the King allowed Belgium to obtain needed gear during the covid pandemic. We can also see how quickly relations can suffer when a bad president is in office. A King means stability.

Finally, the King must know his place. If he steps out of line and expresses the tiniest personal political opinion, he's sidelined real quick. That happened in Belgium when the King refused to sign legalization of abortion. The parliament declared him unable to reign and the law was passed anyway.

It's more like a mascot