2 Man Mechanics FED...

[lawump]

To answer the original post (and assuming the original post was talking about a groundball in the infield where the base umpire would make a call at first, and then have to make a call at third on a runner trying to advance from second base to third base while the ball was being thrown to first base):

For the sake of uniformity our high school association adopted the MiLB two-man mechanics ("the red book"). Since they have the plate umpire remaining at home on a ground ball in the infield, and the base umpire taking both the play at first and the play at third base on the runner attempting to advance...that is how we do it.

As for the theory behind why the base umpire takes the play and the plate umpire stays home: I defer to PBUC (who publishes the "red book") since they eat, sleep and drink umpiring 24/7 for 365 and must have a good reason for adopting this mechanic. Unfortunately, I do not have my red book in front of me as I type this to tell you what their printed theory is.

But to answer the original question: we do it because (1) we need uniformity and (2) that's how they do it at the highest level of baseball umpired by a two umpire crew.