First of all, tithing (giving ten percent) was only commanded under the old covenant.
The old covenant was made only with the ancient Israelites (Ex 19:5-8; 34:27). It was never made with the Egyptians, Assyrians, Phoenicians or any other Gentiles. And then the Israelites broke that covenant anyway (Jer 31:31). Eventually it was superseded for all (Jew and Gentile alike) with the new covenant (Heb 7:22; 8:7,13) that was enacted after Jesus’ sacrifice on the cross (Mt 26:28). As source for commandments, the old covenant is of no use unless those commandments are repeated in the new covenant. In fact, for one to use it to be righteous (i.e. to be justified) in God’s eyes is to fall from grace (Gal 5:4).
In contrast to tithing, there is a type of giving under the old covenant that parallels with that described in the new covenant: the freewill offering (Ex 35:29).
From the heart rather than a specific percentage (Ex 25:2; 2Cor 9:7),
In proportion to one’s blessings from God (Dt 16:10; 1Cor 16:2),
Given according to one’s ability (Ezra 2:68-69; 2Cor 8:12), and
Done in secret without boasting (Am 4:5; Lk 18:12; Mt 6:1-4).
The tithe was like a tax to pay for the services of the Levites and priests (Num 18:21,26-28). Since the Levites made up approximately ten percent of the nation of Israel, such a number was fair and made economic sense.
So you may ask yourself why some preachers still preach tithing from the Old Testament (but ignore other passages like those that command to kill those who break the Sabbath; Ex 35:2). It doesn’t make sense, does it? Well, when things don’t make sense, nine times out of ten it’s about money. Today, when preachers demand a tithe from members, if the church has more than ten families in it, they are looking at a windfall. How do you think those preachers from those mega churches live lives of luxury? If they taught giving as described in the new covenant, they fear having to buy a Kia rather than a Cadillac. However, as mentioned earlier, per Gal 5:4, they preach like that at their peril.