Not wishing to contradict you Jon but surely the idea it's to spread the point load weight of the full bath over a few floorboards rather than on a single board. The floorboards will already distribute the weight over the joists so the feet need to sit on supports that distribute the weight over a couple of boards, especially if the feet are positioned in between joists.
The thought process is ... your current floorboards will already distribute the full bath weight across the joists, this is about distributing the small point of that weight, that the small feet create, across the floorboards.
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