Tithes and Offerings is a primary expression of the Christian discipleship of stewardship. It's cheerfully giving back what was His to begin with.
2 Corinthians 9:6-7 "Whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver."