Ruth 4:5

‘ Then Boaz said, “On the day you buy the field from the hand of Naomi, you must buy it also of Ruth the Moabitess, the wife of the dead, to raise up the name of the dead on his inheritance.”

Then Boaz pointed out what this redemption involved. On the day that he bought the land he would have the responsibility of ‘raising up the name of the dead on his inheritance', by begetting sons through the remaining womenfolk who were selling the land, in this case Naomi and Ruth, the wives of the deceased menfolk. That son would then take the name of the deceased (he would be ‘ben Mahlon, ben Elimelech'). Note that it is made quite clear here that Ruth, though a Moabitess, was now an essential part of Israel with certain rights of land ‘ownership' of YHWH's land. Thus the continual stress on her being a Moabite (not only in his use of the descriptive title, but also in comments continually made - Ruth 2:6; Ruth 2:11) is patently a part of the writer's purpose. He wants to stress that the great King David was descended from a Moabite, and his purpose in this must have been in order to make clear that a foreigner, even a Moabite woman, could become an essential part of Israel (compare how he later describes her seed as ‘the seed which YHWH gives her' - Ruth 4:12).