Grist for/to the mill
The complete saying is that something or other is all grist for (or to) the mill, meaning that everything has a use or a value. It dates in this figurative sense since the 1500s. Grist is an old Anglo-Saxon word from which we get grind and it means the quantity of corn to be ground at the mill. Corn growers rarely owned their own mills so would take their grist to the miller who would do the grinding for a fee.