During a home invasion why do people recommend you announce yourself and warn that you have a gun?

It seems like you are giving up a significant tactical advantage by warning a bad guy that you are aware of their presence and you are armed. I feel by doing this you are sacrificing the advantage of surprise. They now know where you are and what to expect if they confront you. Wouldn't this just raise their guard and make them more apt to attack on sight knowing that they are opposing an armed person? via /r/CCW http://ift.tt/2dDeIKD