Multiple wives would increase the chance of a male heir. A male heir was important for the continuation of the family name and in the case of David, his dynasty. Politically, wives were also a means of securing an alliance with tribes in Israel or nations around Israel. These alliances would help to consolidate power.1
2 And sons were born to David at Hebron: his firstborn was Amnon, of Ahinoam of Jezreel;