It's an interesting one. I like what Jayne says - we give because God is generous and we model God's generosity to the community.
In the OT the whole idea of tithing and Jubilee etc was as much about the community as about the individual. In Malachi God calls the people of God to bring the whole tithe into the storehouse, promising that they will see how he brings abundance to them. In our experience, both in churches and individually, we have found this to be true. When churches give, often beyond what they can, on the surface, afford, God blesses this, over and over.
In the NT the 'tithe' as such is abolished but Jesus' words imply a greater understanding of what giving is. Passages such as the one in Matthew 7 about treasures in heaven, and his conversation with the rich young ruler, show that Jesus expects us to give away. His own ministry was supported by several women who were giving away from their synagogue.
Paul's letters are full of instructions about giving away from the community of faith (the church) to either his mission or, for eg, the poor of Jerusalem. It is clear that he also expects churches to be generous and outward looking in their giving.
I prefer the word 'giving' to 'tithing' but think there is a principle in tithing we shouldn't lose - that of being obedient to God in our giving and then seeing his blessing through this.
I personally would not attend a church which did not give to the poor. I think that a church that gives outwards is a church that grows and a church that sees their own resources multiplied, and I think it's a completely biblical model of giving and generosity.