The extension lets you start the debugging process after you have flashed an application and the application is running on the device. This can be useful in many cases. For example, when using the standard Debug action to debug issues without having to reprogram the application. Another example is when you do not want to use Erase and Flash to Board to flash an application to a device with readback protection enabled.
Use the Attach Debugger to Target action from the Debug's More actions... menu.
Tip
You can also manually edit the launch.json file to launch the debugger against a debugging server that is already running.