The motivations are different: paying down my debt enhances my own financial well-being, while donating to charities enhances others'. Or: paying down debt is self-interested, while donating is selfless (or at least self-interested in a different way).
The point is: living on 0.5 * $annual_income requires discipline. If you give 0.5 to charity, good for you. If you put 0.5 to a retirement plan, or apply to some long-term debt that you assume, good for you. Not everybody has the discipline to get $10 and only look at $5 as "available".
