It's NOT about the preacher!!
Church unity is often wrecked by people who get puffed up in favour of one preacher over another! Paul had to deal with exactly that in Corinth 2000 years ago, and part of his answer was to stick some pins into those puffed up partisans and burst a few of their bubbles.
Much of the trouble comes from a warped view of what ministers are... unduly elevating the preachers.
Paul bursts that bubble by giving three contrasts to show that it really is NOT about the preacher:
1. Preachers are waiters, not chefs.
2. Success is granted, not greatness.
3. Differences must be roles, not rivals.