What is the purpose of a reverse-flow orientation for turbo-prop aircraft?
Are you talking about the reverse flow of the combustor or the reverse flow of a PT-6 installed in a tractor configuration (puller)?

The reverse flow design of a PT-6 using a power turbine separate from the gas core offers several advantages:


simplified gas core and power turbine without the additional complexities of nested spools.


modular gas core, power turbine, propeller gearbox and accessory gearbox for ease of maintenance.
design facilitates ease of installation in a pusher configuration e.g. Beech Starship, Piaggio P180 Avanti without complicated and heavy propeller drive trains.  
Engine exhaust from a pusher configuration additionally provides propeller deicing without additional electrical or chemical deicing boots.
the reverse flow of the tractor layout allows for additional FOD protection vis a vis an intertial separator prior to air entering the first stage of the compressor.  This makes the engine more adept to operating from unpaved airstrips or ones covered with rocks, ice or other debris.
separate gas core and power sections reduce spool inertia in the gas core, making the core - and the engine - more responsive to throttle inputs from the flight crew or autothrottles.