After grabbing the Media Button via this action all presses will be sent to Tasker.
It only makes sense to do this if you configure Tasker to do something with them (via state Media Button) or you want to block them from going to another app.
Note: any app can grab the Media Button at any time, Tasker cannot 'keep hold' of it. For example, the system media player seems to grab the button at the start of each track.