I've never been able to find out why they did this and where this practice started. Where they just horny for someone other than their wife? And was there ever a time where a wife became jealous of the fact her husband had taken on a concubine?
I have some litterature about this at home and will provide them a little bit later.
In pre industrial, or early industrial society the views on family relations and sexuality was very different from today. The concept of family, or rather household could be defined as mother, father, children, grandparents, different kinds of servants and close neighbours. This is what the household in the agricultural society looked like, and I can imagine that the upper classes had a somewhat similar view on family.
Also, there is the question of pregnancy. Men took wives to have a mother for their children, the concept of "love" changed drastically during the industrial era and with the growing burgeouis class. Before this, Marriage was a way to have ligitimate children and pass on a heritage or to make sure that the household survives another generation.
About jealousy, this is hard to know for certain, we do know that the women of early/pre industrial society were supposed to be very restrictive with their sexual life and act in an almost asexual way.
I base my answer on texts by swedish ethnologists Inger Lövkronas text "Gender, Power and Honour".