Responses
Possibly to get support from noble families or keep the noble families in check . If their family concubine is favored it benefits the family so they would help the emperor out more for money and rewards. And if he wants to keep a noble family in check the if the emperor plays their cards right the concubine would foolishly divulge all the family dirt that would back them into a corner and possibly get the whole family killed.
I don’t understand the ML if he loves FL than why did he get so many concubines?