A fielder must tag a runner who is between bases to put out the runner. The fielder must tag the runner with the ball.
To put out a runner on a force play the fielder must have possession of the ball but the fielder need not tag the base with the ball. Any part of the fielder's body will do.
Why is there a difference?
Why not require the fielder to tag the base with the ball?
Or why not allow the fielder to tag the runner with any part of the fielder's body or with either hand?
When a runner is advancing why tag the runner at all? Why not simply allow the fielder to tag the base before the runner arrives for the runner to be out?