I've used many electronic tuning devices over the years and could testify their usefulness from a practical point of view. I started tuning pianos using my ears exclusively but soon realized that an Electronic Tuning Device (ETD) is very useful to evaluate the present state of a piano like its general overall pitch, where or at what region it has gone up or down and of course to know how many tunings it will take to bring it up to standard pitch.
So I got THE machine, namely the Peterson Stroboscopic Tuner that I paid over $2,000 in the 70's! I then sold it and got the VIRTUAL version of that same machine costing way much less at $250. But guess what? I've trashed this machine for an Android App on my Samsung Galaxy SIII for only $10.
Yes, you heard me right, only 10 bucks and it is MORE PRECISE AND ACCURATE than all the expensive machines I've tried so far. Why that you ask?
Technology has evolved tremendously in recent years and the electronics is one of the areas where it shoes most. So the SmartPhone come equipped with very high standard HARDWARE. So the microphone that comes with your SmartPhone is much better than the microphones we use to find in older electronic devices like the Peterson Strobe Tuner. Also, the display is so much clearer than Peterson's old VS-1 (VS=Virtual Strobe) I've used since lately that the readings are much more accurate.
The iStroboSoft by Peterson is no longer available for Android but still available for iPhones. You can still find other tuning apps for Android.