In mine development it is common practice to stockpile material that is below cut$-$off grade but above some other arbitrary lower threshold. For example, in a porphyry copper deposit with a cut$-$off grade of $0\mathrm{.}35\%$ Cu$-$material in the range $0\mathrm{.}250\mathrm{.}35\%$ Cu might be stored for subsequent easy retrieval. Explain why this $$low$-$grade stockpile$$ material might, at some future date, be fed through the mill.