Often times you would have to know the activity name to launch an app from adb. With
monkey command you can launch the app directly using:
adb shell monkey -p com.android.settings -c android.intent.category.LAUNCHER 1
com.android.settings with the package name of the app you would like to launch.
You can use method to circumvent poorly implemented access control on devices that rely on disabling touch gestures and taking over the screen.