@th30wl: yes, but when it comes time for the next generation to take over the throne, or further generations down the line, how do you decide who gets it? Wars over succession were common, but not
that common. And no, the answer wasn't always "the eldest surviving legitimate son takes the throne". In some monarchies, the ruler got to pick from anybody who was qualified — admittedly, "qualified" usually meant "born or married into the royal family", but it still meant that the next ruler didn't have to be the first son.
And then there were cases where there
was no clear blood-line successor: the War of the Spanish Succession happened because Charles II of Spain had no heir (he was too inbred to reproduce — look it up!) and was able to will the kingdom away, and decided to give it to the next King of France in hopes that France and Spain would merge and become a juggernaut mega-country that would rule Europe. (Did not, in fact, happen, as you may have noticed.)