The concept of a king or chief, I believe, is as old as the concept of leadership & so, perhaps, is older than humanity itself seeing as many animal species have leaders. As the human population grew, empires began to form &, thus, emperors. The king (or emperor) was a man who sat around the fire to discuss the land & settle disputes with his advisors (early parliament & government) in antiquity. The king led his soldiers into battle as the monarch of his nation. The king entrusted certain people to ensure the tribe has water & crops, he would gather the best hunters to hunt for meat if there were no domesticated livestock, these were early ministers of agriculture & water security. The king would choose certain men to lead certain regiments (his commanders & generals). The role of a king was very hands on & advanced unlike the largely ceremonial figures we seem to have today.
King Solomon, the king of the United Kingdom of Israel.
If the king grew distant from his people, the people would get rid of him in various ways & a new king would take the throne. The concept of choosing a king or emperor (i. e. a president) for a limited period of time came with the Greeks & was popularized by the Roman Republic. I can't really explain what the reason was for the almost global limit of four or five years in democracies but perhaps it was to ensure a leader stays on his toes & in favour of the people. But the unforseen circumstances of this term limit was that, there is lack of consistency & should the person elected use up his allotment of terms, he no longer can be elected again by the nation/country which may favour him as a leader; whereas a monarch simply has lifetime rule. What's sad today is that one disliked king can bring an end to a country's monarchy if he is out of favour with the people & the monarchy of that country would give way to a democracy.
I am sentimental, I believe a monarch links the government of the land to it's ancient rulers so I believe a monarch has a place in the governance of a land. Seeing as there are problems in democracy & dictatorships, monarchy should not be seen as outdated but as legitimate governance. The monarch himself should be wise as to retain favour with his people & not believe that the kingdom or empire is his personal playground. He is the king by favour of his people as are elected rulers & even though a king may not be voted out, the people have ways to show their disapproval of him. And I trust the world's existing monarchs do not believe that they are above their people. Even though, I believe, monarchs should by default obtain certain privileges to ensure they are in their best state to serve their people, these privileges should not be abused by the monarch as that itself may cause disapproval to form in the nation as with politicians who abuse their positions & steal from state coffers.
Monarchs walk a fine line between being rulers & being servants of the nation. Should a monarch tread on either side of the line too long they would be in danger of being a disliked tyrant or a people-pleasing lunatic. A monarch should seek less to be a monarch & more a people's monarch so he is not seen as a big-headed nuisance or a slave king (a king who is a weak servant of his people).