Adam Clarke on Genesis 14:20

And he gave him tithes - A tenth part of all the spoils he had taken from the confederate kings. These Abram gave as a tribute to the most high God, who, being the possessor of heaven and earth, dispenses all spiritual and temporal favors, and demands the gratitude, and submissive, loving obedience, of all his subjects. Almost all nations of the earth have agreed in giving a tenth part of their property to be employed in religious uses. The tithes were afterwards granted to the Levites for the use of the sanctuary, and the maintenance of themselves and their families, as they had no other inheritance in Israel.