The concept of Kings goes back to early human civilizations when humans first started living together in communities. Certain families would join together in clans, with someone taking leadership of the largest clan, and through that, having influence over the other clans. If the powerful clan was able to maintain its authority, it would gradually develop into a 'court', with high-ranking members of other clans given titles to keep everything in order, and the main clan growing in authority. This clan, and its chieftains, would eventually start giving itself more and more authority until it developed into a Kingship. The 'royal family' aspect was largely used to justify the authority of the clan and discourage any other clans from raising a challenge. As they grew in scale and power, so did their sphere of influence, developing into the royal families we see throughout history. The hereditary aspect of a Kingdom is a commonly seen phenomenon, and generally necessary to ensure the continuation of the King's line. By strictly restricting who could and couldn't marry into a royal line, you were able to strategically align your own line with other lines of power. You didn't want some other family marrying into your line, just to have them try to put their own grandson on the throne.