The New Testament and New Covenent through Christ erased the requirement for a tithe. However, it does say that we should give freely to those who are in need. Furthermore, it also calls us to give everything and follow Christ's example. That is a little tougher standard than the Old Testament requirement, which was actually about 40%. 10 percent to the Levitical Priests. 10% to the government. 10% each year to travel to jerusalme on a pilgramige and 10% once every three years to give to the local homeless and poor in your area.
The most important reason for tithing is that it provides finances for the maintanance and operation of the church. The paying of tithes is how all bills are paid and is how the various ministries of the church are financed. The paying of tithes also teaches one to be more faithful and committed to the work of the Lord.