Tithing honors an Old Testament principle of how God provided for the ministers he called and the expenses of their ministry.
You recall that in the Old Testament God designated one of the twelve tribes of Israel, the tribe of Levi, to be the tribe that would have the ministry of the tabernacle and the temple. So instead of giving them a portion of the land, God said that these vocational ministers of the tabernacle should live off the tithes of the other eleven tribes. In Numbers 18:20–21 God said to Aaron,
You shall have no inheritance in their land, nor own any portion among them; I am your portion and your inheritance among the sons of Israel. And to the sons of Levi, behold, I have given all the tithe in Israel for an inheritance, in return for their service which they perform, the service of the tent of meeting.
When we tithe today, we honor a principle found here. Some of God's people are called not to do moneymaking business in the ordinary ways. They are called to be pastors and ministers and missionaries and ministry assistants, and so on. The rest of God's people (call them "lay ministers") are to be gainfully employed and support the "vocational ministers"—and the costs of that ministry. In the Old Testament God laid down that this be done by tithe.
If the question is raised whether Jesus, in the New Testament, continued this principle for the sake of his church, one of the strongest arguments that he did is Matthew 23:23 where he says,
Woe to you, scribes and Pharisees, hypocrites! For you tithe mint and dill and cumin, and have neglected the weightier provisions of the law: justice and mercy and faithfulness; but these are the things you should have done without neglecting the others.
So Jesus endorses tithing: don't neglect it. It is not as essential as justice love and mercy; but it is to be done.
Yet one might say that he is only talking to Jews in an essentially Old Testament setting. Maybe so. But there is another pointer that the principle was preserved in the early church. In 1 Corinthians 9:13–14 Paul says,
Do you not know that those who perform sacred services [in the temple] eat the food of the temple, and those who attend regularly to the altar [of sacrifice in the temple] have their share with the altar?
In other words he reminds the church that in the Old Testament economy there was this system in which the Levites who worked in the temple lived off the tithes brought to the temple. Then he says in verse 14:
So also the Lord directed those who proclaim the gospel to get their living from the gospel.
The least Paul is saying is that those who spend their lives in the service of the Word of God should be supported by the rest of the Christians. But since he draws attention to the way it was done in the Old Testament as the model, it seems likely that tithing would have been the early Christian guideline, if not mandate.
In other words when we tithe today, we honor a principle and plan of God that sustained the ministry in the Old Testament and probably sustained the New Testament ministry as well.