The additional concubines and wives were likely an attempt by David to cement his relationship with the various tribes in Israel. Marriage bonds were a common way of doing so in the ancient Near East.1 By taking concubines and wives, David behaved in the manner that Samuel had predicted for Israel’s kings (see 1 Samuel 8:13).2
13 And David took more concubines and wives from Jerusalem, after he came from Hebron, and more sons and daughters were born to David.