Giving is a spiritual issue
Giving is a spiritual issue similar to prayer or other disciplines. It's not about what the church needs. It is about our need to give. When we give a percentage of what God provides, it reminds us on a regular basis that our existence, our physical and intellectual capacity to earn a living, are not something we earned. These are gifts from God, entrusted to us to provide what we need in order to live (not everything we want), and what we need in order to serve as disciples of Jesus. Giving reminds us that everything we have comes from God, and makes us thankful people.
Giving makes us content people
When you give back to God regularly, you are thanking God, appreciating what God has given you. Over time, you are no longer worried about what you don't have (like your neighbor's nice car, house, or the latest electronic gadget). You begin to realize that God has given you everything you need already. Giving makes us generous people. Practicing regular giving will transform you into one of those generous people, because you will experience that true fulfillment comes from giving yourself away, just like Christ gave himself for you.
Giving sets us free
We feel compassion for our neighbor who is in need, but we are freed from guilt in that we are already doing what God asks. If you are one of those who is materially blessed, you are freed from wondering why God entrusted so many blessings to you. Like Abraham, you are blessed in order to be a blessing to others, through your giving.