When we had our new bathroom fitted a few years ago, when they sealed siliconed the bath they filled it all the way to the top with water. I asked them why? They replied you need to take into account the weight of a person and the water as over time the weight will eventually pull the silicone away from the wall and obviously break the seal. Certainly makes sense.
You should really have the tub full when you caulk because it flexes the tub slightly.. If you dont, the caulk can pull away from the wall when you take your first bath.
This was already mentioned. I can see why a steel, fiberglass, or acrylic tub mounted on a modern OSB or plywood floor might need this - but this tub is about 200 pounds of solid cast iron sitting on old-growth dense “true” 2x8 floor joists right at the rim joist - it ain’t going anywhere. Silicone caulking remains flexible, and will more than take up the miniscule flexing that it may encounter.
I hate bathroom stuff. Our previous owners didn't use bathroom paint and every few weeks I have to clean the mold off the walls. I need to get around to sanding it down and repainting but it's a chore
