Being an open source platform, Android OS features “developer options” which includes android usb debugging mode. You might have heard people mention it in forums and you may even have seen it while looking through the settings of your Android-powered device. If tuned ON, the developer options help developers to build, test, debug, and package Android apps and mods.
Even if you are an average user and wish to use a computer to root or install a ROM on your Android device, you must turn on USB Debugging mode on your phone or tablet. And you’ll not be able to copy data between your computer and Android device unless you enable Developer Options on your device first.
Programmers that develop Android applications using Android SDK (software development kit) on computers also use USB Debugging Mode to transfer those apps to an Android-powered device for testing.
How To Enable USB Debugging on Droid
On devices running Android 3.2 or older (Android 2.x – 3.x devices):
==> Go to Settings > Application > Development > USB Debugging.
On devices running Android 4.0 and newer (Android 4.x devices):
==> Go to Settings > Developer Options > USB Debugging.
Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options.
It’s best to keep USB Debugging Mode disabled and only enable it when you really need it.