Turrretin:

VI, Now as this calling springs from a threefold principle, so it obtains a threefold end. (1) It springs from the authority of a legislator who has the right to prescribe to man his duty. (2) It springs from the goodness and grace of a Lord who does not cease to bless the creature (although unworthy and guilty) by showing him the way of salvation and showering upon him various blessings. (3) It springs from the justice of a Judge who wishes to convict the stubborn and rebellious and to render them without excuse. Hence a threefold end flows. The first is the prescription of duty that he may know what God demands from him and what he owes to him (namely, to believe and repent). The second is the promise of blessing on the condition that he knows what God has determined to give to believers and penitents. The third is the detection of the wickedness of the heart (Lk. 2:35) and its inexcusability (Jn. 15:22), its stubbornness being supposed, so that both the man himself in his conscience and others may really know that the vengeance of God against that servant is just (who while he knows his master’s will and ought to do it, still neither does nor wishes to do it, Lk. 12:47). Turretin, Institutes, 2:505.

C.f., Turretin on God’s desire.