Here's an example of why we stop and not run into the tag. Runner on third base with 1 out in a 1 run game. Batter hits the ball down the line to first baseman who's set up about half way up the line. Runner takes two steps and stops as runner from 3rd starts toward home. First baseman now has to make a decision; back up and tag the bag is not the right answer. Run up and tag the runner is an option but now the runner from 3rd is about to score and you have another choice - which out do I get? In this situation the throw came home to stop the run from scoring. Safe at home and the batter is now standing on first base after ducking the throw home.