Infielders practice turning 5-4-3 and 6-4-3 double plays in this fielding drill.
The Catcher and Pitcher practice recovering after a wild pitch with runner on third in this drill.
The purpose of this drill is to improve general team defense. Setting a high number of outs required like 9 or 18 or even more can, as the number of outs made grows higher, put real game type pressure on players not to mess up - which will make them that much better in real game situations.
The "Middle Infielder" refers to the second baseman and shortstop, whose positions lie in the middle of the infield. The shortstop is typically the best defensive player on the team.