The PlatformGameInstance exposes some Mobile specific functions to Blueprint. The majority of which being events relating to being able to handle when things like the application (game) is about to be deactivated because of an Incoming Call or SMS for example.
Basically you would use it if you were developing for a Mobile platform.
Take a look at the Engine\Source\Runtime\Engine\Classes\Kismet\BlueprintPlatformLibrary.h for more information on how it is set out.