It's just another means of doing the same thing, it creates a compact and light engine arrangement. Note also that the compressor section is also centrifugal flow, again, in the PT-6's case for compactness. An axial flow compressor takes up space. In most (not all) small helicopter and aeroplane turbines the compressor is centrifugal. In the PT-6 the coupling between the turbine and RGB/prop is short, because in every gas turbine there are mechanical inefficiencies in the extraction of energy from the combustor section by the turbine (this is why the turbine needs very hot gases, to maximise efficiency - high temps = high efficiency), so with a reverse flow engine the length of shafts etc can be kept short, which produces a compact yet efficient engine. Note also that the combustion chamber is designed so the gases double back on themselves before they exit the combustor section.
PT-6s are extraordinarily efficient for their size and have enormous application. Take a look at this page, which gives you a list of the applications of this engine.
Pratt & Whitney Canada PT6 - Wikipedia