Adam Clarke on Deuteronomy 14:29

And the Levite (because he hath no part nor inheritance - And hence much of his support depended on the mere freewill-offerings of the people. God chose to make his ministers thus dependent on the people, that they might be induced (among other motives) to labor for their spiritual profiting, that the people, thus blessed under their ministry, might feel it their duty and privilege to support and render them comfortable.