Bathing is in this vocabulary because the electional tradition put it there. Medieval and Renaissance election texts carry lists of ordinary bodily acts with a recommended moment for each: cutting the hair and nails, taking medicine, letting blood, and washing. They are the most domestic material astrology ever produced, and they are entirely serious in their own terms.
The Moon carries almost all of it. In the humoral physiology these texts assume, the Moon governs the moist and the cold, the tides of the body, the phlegm, and everything that increases or decreases on a cycle. A body is treated as a thing with water in it, and the Moon is what the water answers to. So a bath is timed by the Moon's sign and phase before anything else. The water signs, Cancer, Scorpio and Pisces, are the obvious ones, and the waxing Moon is used where the intention is to increase, the waning where it is to draw off.
Venus enters as the planet of ease, adornment and the pleasant condition of the body, which is what distinguishes a bath from a purge. Where the Moon supplies the physiology, Venus supplies the reason a person would want one.
None of this is medical advice and the atlas does not offer it as any. It is a record of how one tradition reasoned about the body, and the reasoning stays legible even where the physiology behind it has long been superseded.