The heirs to the throne are more experienced and prepared to rule and lead as from a young age, they are taught the ways of a good ruler.
Leaders have more freedom within a Monarchy:
As a part of a monarchy, you have the freedom to make your own laws. Although, in a republic, all of the other members have a fair say and a majority must agree to the leader's proposals.
The people have no say in who is ruling their country. Also, if the heir to the throne is not competent for the role as leader, it is not possible within a monarchy to re-elect someone who be a more effective leader.
Excessive Power invested on a single persona:
The monarchs are the supreme judicator, legislator and executor and though they are assisted by their advisors, it will still be them who will have a final say in things.