The food at foodbanks is provided free.
The idea is to make the food last longer and get the maximum nutrition from it, and so that if you have Ł10 or Ł5 or Ł3 spare, that you can make the most out of that.
Quite obviously, helping people who are hungry to maximise the use of food is a good thing, even if it it within the context of foodbanks generally being a negative.