In our Western society, we think of democracy as the only legitimate form of government. However, when a king provides protection and leadership for his people and also serves his people with good governance, then we have a society in which members can prosper.
That is the kind of kingship David brought to the Jewish people. The Lord said to David, "You shall shepherd my people Israel and shall be commander of Israel." Really, the Lord was their king, and David was the Lord's spokesperson.