Making it up as I go along

So here’s what I don’t get

You have a football team.  They’re going to a major tournament.  They know they need to practise penalties in case of a penalty shoot-out.

So the outfield players need to be good at getting the ball past the goalkeeper, while the keeper needs to be good at stopping the ball from getting past him.

So they practise by having the players take penalties against the keeper.  Which will result in either the player scoring (in which case the keeper’s doing a crap job) or the keeper saving the ball (in which case the player’s doing a crap job).

Seriously, how on earth can that possibly work?!