If you want to use Android SDK on your Android phone, you have to enable USB debugging mode on it. Since you need to access the Android SDK if you want to root your Android phone or install a custom firmware on your Android device, you may have to first enable USB debugging on it.
There are also some Android apps, like synchronization apps, that needs USB debug mode enabled to perform file operations on your computer connected to your Android phone via USB.
Since most users will never need to enable USB debugging mode, no details are available on it in your Android phone or tablet manual. Also, by default the USB debugging mode is disabled on all Android phones and devices. But its really easy to enable it. Here’s the procedure.
First, if your Android phone is connected to your computer via USB cable, please unplug the USB cable from phone. Its not sufficient to simply disconnect the cable from your computer, you must disconnect it from your phone.
How To Enable USB Debugging in an Older Android Phone
To enable USB debugging in Android phones running any older version of Android OS (< Android 4), click on 'Settings > Applications > Development’ and check the USB debugging option. The phone will ask if you really want to enable it? Confirm to enable USB debug mode.
How To Enable USB Debugging in Android 4.0 Phones
To enable USB debugging in Android phones running Android 4.0 Ice Cream Sandwich, go to ‘Settings > Developer options’ and check USB debugging option. Your Android phone will ask if you really want to enable it? Confirm to enable USB debug mode.
Update: With Android 4.2 Jelly Bean, the developer options and settings are hidden and not easily accessible. Check the procedure to enable developer settings in Android 4.2 Jelly Bean here.
Important Note: Its recommended to disable the USB debugging mode on your Android phone or device as soon as you are done with the task associated with it. Keeping it enabled is a huge security issue as it can be exploited to install malware on your Android phone or to access private data stored on your Android phone.
Alternative to Enabling USB Debugging Mode on Your Android Phone
If you can’t enable USB debugging mode on your Android phone for some reason or if you don’t want to enable it for security reasons, then you can still perform most operations on your Android phone that USB debugging mode will enable you to do.
You will need to use the phone software provided by your phone vendor to do this. For example, you can use ‘HTC Sync’ for HTC phones, Kies for Samsung phones, and PC companion for Sony Ericsson phones.