Why does Paul use Christ's example when talking about Christian giving?
Paul uses Christ's example to call believers to the same self-humbling, sacrificial attitude in Christian giving. Since Christ willingly left His riches, became poor, and took the role of a slave for salvation, no believer can rightly claim inability or unwillingness to give. Giving should flow from readiness of will and performance according to what one has.
Les explains that Paul asks why he is using Christ's example in relation to Christian giving. The point is that believers must take the same kind of attitude. If God in the person of Christ was willing to leave that area of absolute control of the universe and took on Himself the role of a slave for our salvation, then who are we to say, "I can't give anything," or "I can't give so much"? In II Corinthians 8, Paul gives his advice because it is expedient and good for them. They had begun before, not only to do, but also to be zealous a year earlier. Now they should perform the doing of it: as there was a readiness to will, so there may be a completion or performance out of what they have.