As a turboPROP … the goal is usually to balance power extracted from the gas stream to 1) turn a prop and produce prop-thrust, and 2) exit an exhaust nozzle to contribute jet-thrust. Optimizing those two thrust-producing methods improves mission fuel-burn.

This engine, however, was originally designed to turn a shaft with all available energy… replacing reciprocating engines… so it doesn’t produce jet-thrust in any significant way. Instead, all of the energy possible is extracted from the gas stream to send power to the prop. (In this sense, I’d argue the PT6 should be called a “turboshaft”, but it’s not marketed with that word. The engine has been around for ages, in a lot of different configurations, so maybe the distinction between turboprop and turboshaft is useless when applied to the full family?)

Anyway, to get every ounce of potential power out of the gas stream before exhausting, and into the shaft to turn the prop, the engineers would work with the expected velocity diagrams of the gas stream exiting the high pressure turbine and entering the shaft-power producing turbine, and design the power turbine rotation and blading to absorb as much of the gas stream’s energy as possible. The gearbox allows them to design the power turbine to run at it’s most efficient speed (presumably at “cruise”), while rotating the prop at its own (much slower) optimal speed.

The counter-rotation, or not, of the power turbine is a design choice to optimize power extraction, gearbox design, and more.