Scott,

SWAG, which has probably already occurred to you:  tongue and grove on heavy
wooden flooring for which no pre-milled T&G was available.

As you’ve inferred, it has to be for an extensive, repetitive task on thick
stock.

Think of riven (split-out) or crudely-sawn (pit-sawn?) boards, planed on the top
surface, then T&G’d by hand.  The bottoms of the boards won’t rest equally on
the beams until the carpenter cuts a dado across each board in way of the beams,
usually with a lipped adze.

Or, it could be for working on boards with ingrained abrasives, like Teak.

John Ruth