I'm not saying that food banks never pay full price for food-- I'm sure it happens some. But for the most part food banks pay way below market rate.
But, for example, if you make a >$100 donation to Second Harvest Food Bank (i.e. so that transactional costs are small), each 50 cents becomes a distributed meal. Note that they collect a few additional cents from the partner charity that is distributing the meal.
OTOH, the school I work at does a Thanksgiving meal drive where students buy food at retail and bring it in. It is definitely less efficient than giving funds somewhere like SHFB, but I think it's an important tangible experience especially for younger kids to give something they recognize as food to the less fortunate.
Well this food bank does. So it would be silly to claim otherwise :-p.
I'm not saying all food banks should just do vouchers. I'm saying that if you're at the point of paying retail prices, you may as well give someone a voucher rather than spending time, effort and money on a tin of beans that a person doesn't even like.
The point of my example is that they could pretty easily do better for those that they're trying to help, but that would involve doing less themselves. Which demonstrates that it isn't all about helping others, it's about demonstrating that your helping others.