God's economics are set out in 9:6: whoever sows sparingly reaps sparingly. Whoever sows generously reaps generously. It is the opposite of the way the world thinks, which is: sow sparingly so you will keep as much as possible for yourself. To the ears of the world, 9:6 sounds like craziness: "throw your money away on those who don't have any so that you will reap."

But is not just financial economics. Follow what Paul says:

We should sow generously, giving cheerfully
This means we will reap generously
God will "enlarge the harvest of your righteousness" (v10)
You will be further enriched in order that you can be generous on every occasion (v11)
So that glory goes to God, from those he has blessed through you (v11)

It seems that the Corinthians have a question mark over this. Namely, why? The answer is v8. God has made, is making and will make grace overflow to them precisely for the purpose of them abounding in good works. We aren't meant to keep grace for ourselves. It has overflowed to us in abundant measure. Therefore it is meant to overflow from us to others. In other words, he gives us grace so that we can grace others. He is inviting us to be like him, in miniature.

That's why giving enriches us. v13 says that people will praise God for the obedience that accompanies the Corinthians' faith. And v14 says that their hearts will be expanded in prayer for them because God has given them surpassing grace in order to be able to grace others. They are becoming like him. Their instincts for gracious generosity are being modelled after his. Their desire to bless is starting to look like his. Their hearts are being enlarged to expect more gracious provision from God, and educated to know how to pass it on when he gives it. They are growing out of that selfishness that marks out spiritual immaturity.