When you first read 2 Corinthians 8, it's easy to miss Paul's point. It appears to focus on the grace shown by the Macedonian church to other needy saints. In fact, it's the other way around. In 8:1, Paul explains that he writes "about the grace of God that has been given to the churches of Macedonia".
Verse 2 explains more: "...for in a severe test of affliction, their abundance of joy AND their extreme poverty overflowed in a wealth of generosity on their part." It was God's grace not only that they would have the joy to be generous but the poverty as well. To miss this would (1) ignore the conjunction "and" first of all, but (2) to miss the entire context and argument of the passage. Paul says to the Corinthians that their difficult circumstances was a grace from God in order that their dedication to God would be manifest(:5) in genuine love (:8), exemplifying Christ's gospel (:9) so that, ultimately, God may be glorified and the needs of the saints met (9:12-15). It's the poverty of the circumstances that Paul's stresses in order to make clear how much more God is glorified in their love and faith which would otherwise be encumbered by an abundance of wealth. (This of course does not make wealth evil or useless, but this certainly challenges many Christian's self-justifying ambitions for wealth.)
Paul's citation of Ex. 16:18 in 8:15 (recalling the manna story) explains in part why the Corinthians and Macedonians could dare to be so bold in love--because they trusted God to fulfill His promises to provide as they needed, not necessarily all they wanted. In short, their giving was an act of faith in God's future grace. God satisfies!
How quickly most of us are to spurn such "grace", yet this is the whole thesis of Philippians, not to mention other passages.
The point of Paul's words is not only that God had granted them this heart to give, but that the circumstances enabled them to magnify God in the service of others. Moreover, dire circumstances teach us the difference between our professed desires (8:10-11a) and actually living out our faith (8:11b-12). We can boast of love, knowledge, and faith in comfortable times, yet affliction tests genuineness.
Finally, this passage reminds me of something John Piper has somewhere written--that we must be cured of our allusive drive to pay back our debt to God. Our good works pay off not debt. In fact, genuinely godly works put us in more debt due to the grace required from God that they be done. In other words, we should seek more grace from God and therefore go more in debt to Him. Nothing glorifies him more than infinitely increasing our debt to Him through desperate dependence!