There were several reasons a person might want to enter into such a relationship rather than marriage. For a woman, this often meant some increase in status and wealth, even when it wasn’t permanent. Her future was reasonably well assured and she got to keep the gifts her patron gave her. There was also the possibility that her patron might eventually marry her, as happened to Angelberga, though this may have been somewhat more likely when both concubine and patron were of the nobility. Her family might also encourage such a relationship, not only because any increase in her status enhanced theirs, but also because a concubine required no dowry. However, she also suffered several legal disabilities. She inherited very little if her patron died intestate, her children were illegitimate, and her patron could abandon her much more easily than he could a wife.
On the patron’s side of things, he got the status and the assumption of stability that came with having a long-term monogamous* relationship with an easier out than marriage gave him. He was also less restricted in his choice of partner, though politics would still have had a bearing on it if he were of the nobility. Since a concubine’s children were not legitimate unless their father married their mother, they did not lessen the inheritance of his legitimate children much, if at all.**
The thing is, it was often difficult to tell the difference between a concubine and a wife. Sometimes the couple themselves may not even have had a clear idea, especially in the lower classes. By the 12th century, priests’ wives were concubines legally but wives socially. Canon law on consent and secret marriage meant that if the issue ever came to the court, it could easily come down to one person’s word against another’s and people could easily find themselves with a marital status quite different from what they had assumed.
*Usually monogamous anyway. There were exceptions and not just one-man-two-women exceptions.
**In some places and times a concubine and her children inherited a smaller portion than legitimate wives and children. In others, they got nothing but the gifts he had already given them and anything he left them in his will if he had one.
Brundage, James A. Law, Sex, and Christian Society in Medieval Europe. Chicago: University of Chicago Press, 1987.
Brundage, James A. "Concubinage and Marriage in Medieval Canon Law." Journal of Medieval History (1975): 1-17.
Dübeck, Inger. "Women, Weddings and Concubines in Medieval Danish Law." Scandinavian Journal of History 17.4 (1992): 315-322.
Rawson, Beryl. “Roman Concubinage and other De Facto Marriages.” Transactions of the American Philological Association 104 (1974): 279-305.
Marriage Canons in The Decretum of Gratian - Catholic University of America