Surrender your principal advantages, which are concealment and preplanned field of fire.
But you trade it for stopping power. No one who has been hit with a shotgun blast indoors will ever return fire.
Despite what someone posted above, it is also useful for scaring off a prowler lurking in the yard who hasn't made contact with the house yet, be it a local kid, a lost drunk or a serious thug. Open a door or window a safe distance from the trespasser and give the 870 a firm, slow pump. An old man I met years ago who lived in a really bad inner city neighborhood told me that it has worked dozens of times for him and his neighbors.