It was a surprise for some when different applications were designed to provide substitutions for very common hardware devices. If the smartphone has a precise microphone and at the same time is plenty of computational power to compute even the FFT in real time, a viable substitution is a device for having on the go a method to tune instruments. Some applications are simply better according to our tests than a dedicated device for the purpose and it has explanation.
Light meters are another common tool for photography or video footage, with some units having high prices due to their accuracy. It's another sensor that some devices bundle, so it happens in a comparable way.
Video or photo cameras, audio recording —maybe with additional hardware—, GPS, radio manufacturers or others in a long list are experiencing problems when users find a substitution with enough functions for their needs, right?
Now, it seems the time for studying those needs and provide better functions! As smartphones evolve too, it's a difficult task.