It is easy to misread saucha as fastidiousness, and some of its history invites the misreading: purity rules hardened, in classical India as everywhere, into instruments of social exclusion, and the vocabulary of cleanness was made to do work that had nothing to do with yoga. Patanjali's own frame is narrower and more functional.
In the Yoga Sutras purity is preparation: the first niyama because it is the precondition of the other four, as a laboratory is cleaned not out of squeamishness but because contaminated instruments give false results. Contentment is unstable in a disordered life; austerity practiced by an unclean body turns to illness rather than refinement; study requires a settled place; surrender requires a mind not churning with concealments. The fruit Patanjali assigns to mental purity, the sequence running cheerfulness, one-pointedness, sense-mastery, vision of the self, is precisely a description of an instrument coming into calibration.
Later devotional traditions added a warmer register: the body as temple, cleaned as an act of reverence toward what it houses. The two registers agree in practice, and both are far from mere hygiene. Saucha asks what in one's life, physical or mental, is currently distorting the readings, and treats the removal of that distortion, patiently and daily, as itself a spiritual act.