John Calvin on Deuteronomy 18:3-5

3. And this shall be the priests’ due. It is not only for the sake of the priests that God enumerates what He would have them receive, that they may obtain what is their own without murmuring or dispute; but He also has regard to the people, lest the priests should basely and greedily take more than their due; which sacred history relates to have been done by the sons of Eli, (1 Samuel 2:23,) for they had advanced to such a degree of licentiousness, that, like robbers, they seized violently on whatever their lust desired. Lest therefore they should give way to this gross covetousness, God prescribes to them certain limits, to which they were to confine themselves, so that if they transgressed them, it was easy for any of the people to convict them of avarice.