Everything that follows in these two chapters is driven by the first two verses:
You are the children of the Lord your God . . . for you are a people holy to the Lord your God. Out of all the peoples on the face of the earth, the Lord has chosen you to be his treasured possession.
The first part of chapter fourteen following this is rules for what the Israelites can eat. Paul Copan in Is God a Moral Monster gives a great explanation, a plausible one as to why God would do this. It seems random, but if God commands it, it is not.
Then Moses explains the tithe, and why the people are to set aside a tenth of all their fields produce, but the purpose is to eat that tenth in the presence of God, at his dwelling. The purpose? “That you may learn to revere the Lord your God always . . . and rejoice.” God takes rejoicing seriously. When he blesses we are commanded to enjoy! Strange that this would be necessary, but our hearts tend toward complaint and envy and everything else wicked and self-centered. He wants our eyes, hearts and minds focused on him, for in him we find our fulfillment.
The last two verses of the chapter show how radical and revolutionary the religion of the Hebrews was:
28 At the end of every three years, bring all the tithes of that year’s produce and store it in your towns, 29 so that the Levites (who have no allotment or inheritance of their own) and the foreigners, the fatherless and the widows who live in your towns may come and eat and be satisfied, and so that the Lord your God may bless you in all the work of your hands.
There is simply no other religion or worldview in the ancient world that had any ethic for the downtrodden or outcasts. Notice that God’s blessing is directly related to how they treat these people. Amazing. If you look to the “enlightened” Greco-Roman world, this mentality simply doesn’t exist. Add to this Jesus teaching that we should do something as absurd as loving our enemies, and Western civilization was changed forever. This completely new, out of the ordinary ethic is yet more evidence that the OT is truly a revelation of God and not made up by man.