What is the primary cause necessitating Ghusl (ritual bath)?

Primary text

The true necessitate for Ghusl is menstruation (Hayd) and post-natal bleeding (Nifas), as these constitute the major ritual impurity (Hadath). The cessation of these discharges is the condition for the obligation and validity of the Ghusl. This is analogous to the ruling that the cessation of Istihada bleeding invalidates the prayer; the invalidator is the exiting impurity, but it is excused due to necessity. When the bleeding stops, the necessity ceases, and the ruling related to the impurity becomes apparent, with the ruling then attributed to the cessation due to its timing.