You use the TargetPath with the windows adapter when you have the StartMethod set to "StartAndWait". You'd use this for an application that uses a "launcher application". For example; in Windows 10, "Calc.exe" is a launcher for "Calculator.exe". If you used the StartMethod of "Start" and set the Path to "Calc.exe", you would not be able to interrogate "Calculator.exe" since Pega Robotics would be hooked into Calc.exe (which launches and closes). StartAndWait allows you to launch an application (specified by the Path and Arguments properties) and wait for that application to start and inject into it (using the TargetPath property).
***FYI Calculator is a bad example as in Windows 10, you can't really interrogate it anyway, but you can see the idea