Needs demand it. There is much physical and spiritual hunger in the world today. Millions of people are starving. Even more am without Christ and without hope. Love constrains it.
Results justify it.
The results of tithing are both material and spiritual. To get where you want to go you can't only do what you like. In today's lesson Paul is trying to make it clear that it is only through God's grace and the power of the holy spirit that mankind can be saved. Thus he tells the members of the church in Corinth that he does not use an extensive vocabulary or dramatic speech in addressing them. He wants them to understand that the speaker is not the important one for them to follow, but to let the spirit have its way and draw them closer to god.