1.) We give now because He gave first.
John 3:16 For God so loved the world, that He gave his only Son, that whoever believes in him should not perish but have eternal life.
2.) Scripture lead us to give & returns our investment for the good of the local Church.
Malachi 3:10 Bring the whole tithe into the storehouse, that there may be food in my house. Test me in this,” says the Lord Almighty, “and see if I will not throw open the floodgates of heaven and pour out so much blessing that there will not be room enough to store it.