Why did emperors in ancient China have to marry three wives and six concubines?

The ancients paid attention to carrying on the family line, with hundreds of descendants, and so on, and married more sons. In ancient times, civilians could marry three wives and four concubines, not to mention officials. As for the emperor, the world belongs to him, and it's nothing to marry more wives. There is another reason. If you have more wives, you will have more sons. The medical level of ancient people was limited and the survival rate was not high. If you have more children, the survival rate can be higher.

Please accept it, thank you!