There are a number of different ways it can benefit someone, depending on their particular tax situation. The key is that, if you get a check from your IRA & then stroke a similar check to the church, the check you GOT from the IRA is included in your adjusted gross income. That can be bad for a lot of calculations. If you're in the income range where more AGI causes you to pay a extra for medicare, you've just added more to that AGI & maybe pushed yourself into the next higher bracket of medicare premiums. If your income is lower, you might be in the range where a portion of your SS benefits are taxed. In that range, it's maybe even worse - because every dollar of additional AGI makes another dollar of the SS benefits taxable. If you're higher income & have certain other itemized deductions, they can get limited or decreased based on your higher AGI.
For a lot of others, it's simpler. If you don't itemize, you've added the required distribution to your AGI but don't get any deduction because you use the standard deduction anyway. Quick example; without the RMD your income is $50k. Married, subtract the standard deduction of $24k, your taxable income is $26k. Now, assume this year everything's the same but you have to take a $6k RMD. If you take the check, your income's $56k, subtract the standard deduction, taxable income is $32k. Assume that you always donate $5k to your church. In the above, you don't get a deduction for it because you use the standard deduction.
Now, change things so that instead you do the $5k to the church using the QCD rule. IRA sends $5k to the church, you get the other $1k for bourbon money. The money that went to the church is disregarded when it comes to your tax return. You've only added the $1000 to your income (your income would now be the original $50k + $1k from the RMD, minus the standard deduction you're taxed on $27k. You ended up in the same cash situation, but save the tax on the amount of the QCD.
