The GCWR is the important number as it is the total load that the engine, drivetrain, suspension, frame, brakes, and other components can safely handle. Too high a tow load and the engine and transmission will suffer and have a shorter useful life.
When GM in 2011 and Ram in 2013 greatly increased the tow rating for their heavy duty trucks (2500 and 3500 models) they beefed up the frames, the suspension, and the brakes, as well as increasing the horsepower from the engines. There is always a weakest link and the GCWR calculation incorporates at all the potential points of wear and failure for the vehicle.
Tow ratings though are fictitious in many respects. A vehicle rated for 10,000 lbs. may come with a hitch rated for 7500 lbs., or gearing that is too high, or brakes that are not capable of stopping the vehicle without the use of the trailer's brakes as well, or the ATF may operate at too high a temperature with extended towing up steep grades or at high elevations or with high ambient air temperatures.
Big difference as well in regularly towing a load over the Rockies in the summertime and pulling the same load across hill country or flat lands for a couple weeks out of the year. The 3/4 ton is a better choice for the first set of conditions and a 1/2 ton may be adequate for the second set.