It's a 4th outfielder sometimes used in pre-little league youth baseball and occasionally, in slow pitch softball (moreso in pick up games at picnics and so forth, not so much in leagues.) Think of a family reunion where the outfield consists of your out of shape middle-aged uncles, who haven't exercised in years…they're probably not gonna be too fast getting to the ball. Having a 4th body out there will help keep scoring down a bit, and make the game move a little quicker. The same goes for 7 and 8 year old kids, plus it allows one more kid per team to be on the field getting some fielding and throwing experience in game situations.