This is if your have one common for the front yard and one for the front, or if you don't want to make a splice in the field before the timer. This is a convenient option, but most people don't over think it. It is a wire option that is nice because some times the wire is heavy gauge and you can't put 2 commons on the terminal strip because your wire is too thick.
If you are residential install, do what is convenient. But pay attention to wire sizing charts!!
Thanks