I believe the reason many people recommend using GVWR to ensure one has adequate capacity with their TV, is it would represent the potential worse case weight of the trailer, when loaded.
I will freely admit that it is harder to fill up a trailer with 3000 lbs cargo capacity vs one with 1000, but if your truck can handle a trailers GVWR, then it can handle anything lower than that, likely comfortably.
Actuals are obviously the best, and knowing both is obviously the goal. But, when trying to decide if a truck is sufficient for a trailer BEFORE PURCHASING IT, using the GVWR as worse case makes sense.
Mike