@Frameworks Engineer : There are countless usecase to want to launch the watch app, once the iPhone Companion is started, or when iPhone app gets in certain state that requires the watch to be used. Vise versa also. There could very well be reasons to launch the iPhone app when AppleWatch app is launched or certain state becomes active.
My usecase: I am developing an app where the watch is used to manually count certain events via the watch. The iPhone app is used as for log of the events.