One of the unfortunate effects of human sin and of the weird and fractured borderline between the sacred and the secular in postmodern culture is that the word "tithe" provokes reactions ranging from the skeptic's cry "The whole thing's a scam!" to the dim uncomfortable notion of many people that money, if it doesn't belong to the devil, is at any rate coated with icky microbes from the infernal regions and ought not to sully the life of the Church.
But neither of these views reflect the biblical perspective on tithing and our stewardship of money. To be sure, the love of money is called the root of all kinds of evil by St. Paul (not unlike the love of food, power, sex or any other creature above God). But money itself is, like all created things, a gift which we are responsible to use rightly. And there are right uses for money, among them the tithe.
What is a tithe? It comes from the Anglo-Saxon word teotha, meaning a tenth. Generally, it has been defined as "the tenth part of the increase arising from the profits of land and stock, allotted to the clergy for their support or devoted to religious or charitable uses". However, there have been tithes long before there were "clergy" in the Christian sense. So a more biblical definition is "the tenth part of all fruits and profits justly acquired, owed to God in recognition of his supreme dominion over man, and to be paid to the ministers of the church".
Tithing is found in remotest antiquity. It predates both Christianity and Judaism and was a common practice among all ancient peoples. Lydians paid a tithe of cattle to their gods; Arabians paid a tithe of incense to the god Sabis; and Carthaginians brought tithes to Melkarth, the god of Tyre. Genesis mentions tithing, not as a new idea, but as something done by everybody for time out of mind. Abraham is enacting an already ancient custom when he tithes to the royal priest, Melchizedek. Likewise, Jacob is recorded as giving a tithe of all his possessions to the Lord. Mosaic Law the made tithes obligatory. Israel was commanded to offer to God the tenth part of the produce of the fields, of the fruits of the trees, and the firstborn of oxen and of sheep (Lev. 27:30; Deut. 14:22) as well as other forms of tithe.
Significantly, God is seen as the true recipient of the tithe offering. In the Old Testament the idea is that everything belongs to God, but that he, in his generosity, lets us keep 90% of it. The 10% represents the whole. And even the 10% is not destroyed (as in a whole burnt offering) but is instead used to support the priestly tribe (the Levites, who have no property of their own), and strangers, widows and orphans. This sacred dimension of tithing cannot be emphasized enough. Tithing was seen in Scripture, not simply as a handy way to keep the priests paid and the poor fed, but as a debt to God and an elementary aspect of duty to him. That is why, when Israel grew lax in its payment of the tithe, it was seen, not as a neglect of the poor or the priest, but as an affront to the Almighty himself. As Malachi 3:8-10 says:
"Will man rob God? Yet you are robbing me. But you say, 'How are we robbing thee?' In your tithes and offerings. You are cursed with a curse, for you are robbing me; the whole nation of you. Bring the full tithes into the storehouse, that there may be food in my house; and thereby put me to the test, says the Lord of hosts, if I will not open the windows of heaven for you and pour down for you an overflowing blessing."
To rob God is to rob his servants and his beloved poor and to rob them is to rob God. Conversely, generosity in tithing is identified with a blessing so full and sure that God (in a rare moment) actually bids his people "test" him to see how he will bless them.
This identification of God with his priestly servants and with the poor continues in the New Testament. It is already a precept in the New Testament that "the worker is worth is wages" and that the ministers of the altar, having no other means of support, should live by the altar (1 Cor., 9:13). Likewise, Paul takes up a voluntary collection for the poverty stricken members of the Church at Jerusalem. As the Church grows and becomes more organized (as well as taking on the burden of an increasing number of charitable institutions ranging from hospitals to various religious orders to orphanages), the need grows for legislated tithing, not unlike the Mosaic tithe and this became the norm for several centuries.
That said, the fact remains that, in the present era, tithing is again largely left up to the conscience of the believer. The point is not meeting a legalistic requirement, it is cultivating a heart of generosity like Christ's. Tithes are one way to do this. There are others. That is why the Catechism of the Catholic Church emphasizes, not a particular amount or method of tithing, but the general demand of generosity:
2449. Beginning with the Old Testament, all kinds of juridical measures (the jubilee year of forgiveness of debts, prohibition of loans at interest and the keeping of collateral, the obligation to tithe, the daily payment of the day-laborer, the right to glean vines and fields) answer the exhortation of Deuteronomy: "For the poor will never cease out of the land; therefore I command you, 'You shall open wide your hand to your brother, to the needy and to the poor in the land.'''[Deut 15:11 .] Jesus makes these words his own: "The poor you always have with you, but you do not always have me."[Jn 12:8 .] In so doing he does not soften the vehemence of former oracles against "buying the poor for silver and the needy for a pair of sandals . . .," but invites us to recognize his own presence in the poor who are his brethren:[Am 8:6; cf. Mt 25:40.]
The key words in the passage above are "juridical measures". Tithing 10% of our income (or whatever we can reasonably afford) is one very sensible way of obeying the command to "open wide your hand to your brother, to the needy and to the poor in the land". "Your brother" includes not only the priests, religious and needs of the Church, but the myriad charitable works She undertakes and all those in need. "The needy and the poor in the land" include every last one of the "least of these" who are, whether we realize it or not, Christ in disguise.
And that, in the end, is the answer to the question of "why?". For the fact is that in tithing we are giving to Christ (or rather, giving back to Christ). It's as simple as that.