The point is this: whoever sows sparingly will also reap sparingly, and whoever sows bountifully will also reap bountifully. Each one must give as he has decided in his heart, not reluctantly or under compulsion, for God loves a cheerful giver. And God is able to make all grace abound to you, so that having all sufficiency in all things at all times, you may abound in every good work.
— 2 Corinthians 9:6–8

One important reason that we ask for God to grow our capacity is that, through our giving, God brings about many good deeds for his glory. Paul says at the end of verse 8 that when you sow bountifully and cheerfully, you will have an abundance for what? For every good work. We are given an abundance to facilitate our good deeds. This should be the desire for our excess. Our good deeds shine a light that points people to the glory of God. If we lay all of our treasures here on earth, then people will have no reason to think our Father in heaven has any value. We don’t want to live a life that looks like we love what everyone else loves.

[Christ] gave himself for us to redeem us from all lawlessness and to purify for himself a people for his own possession who are zealous for good works.
— Titus 2:14

We ask God for a greater capacity so that we can give it away and show where our true treasure lies. This brings glory to God and lays the way to extend his kingdom.

Jesus Christ himself laid the groundwork for why and how we should give:

For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich.
— 2 Corinthians 8:9

This means that we should give in line with the amount that we love. Charles Spurgeon described this verse by saying that we know how much our Lord Jesus Christ loved by knowing how much he gave. He gave himself to us because he loved us with all of his force and nature. May we give as we love, and measure our love by our gift.