Deuteronomy 18:5
For the LORD thy God hath chosen him out of all thy tribes, to stand to minister in the name of the LORD, him and his sons for ever. For the LORD thy God hath chosen him out of all thy tribes, to stand to minister in the name of the LORD, him and his sons for ever.
The Lord chose the priest out of all the tribes of Israel for sacred service. He was appointed to stand and minister in the name of the Lord. That choosing upheld the plan for the priests to be sustained by the people they served, including through tithes.