Responses
Well, there are different cases. Umm, firstly when the empress is infertile or cannot make an "heir"(boy). The king would be forced to find another woman. Unless the king is just a plain sex crazed bitch.
It could also happen because of political machinations as well, like a method of checks and balances by marrying women from different families. That’s what i saw from different stories and not all of them are sex crazed.
Then why the heck all this time emperors hve concubines. All problems rise from there. They shouldnt have that much women and they should normalize one family. The king and people at fault for okay about this kind of things. They should have stopped seeing women as just birth machines. Than all this crown fights didnt happen. But of course men have to first satisfaied their di**s.