What is the explanation of the Usuli rule in the chapter of Necessities: "Its consideration should not invalidate the origin," along with examples?

The jurists of Islamic legal theory (Usuliyyun) have stated that a legal cause (illah) may particularize or generalize its original source from which it was derived. However, it cannot invalidate that source, because invalidating its original source—which is its very origin—would be tantamount to invalidating the cause itself.