Sometimes you need APK file from the Android device. Easiest way is to use ADB. You need to connect android device to computer and have the required drivers of Android device installed along with Android SDK tools.
Run below command to display all the packages installed on your device.
adb shell pm list packages
Find a package for which you need the APK and run below command to find the path of APK
adb shell pm path package.name
Now pull the APK file to your computer using below command. This will save APK in the same folder from where you started Command Prompt.
adb pull /data/app/package.name