Locate the setter does several things for you.
In serve receive, the setter never takes a pass, so locating the setter will tell you tell you whether or not you have to watch for them to be back-row attackers or back-row blockers. If anyone is going to chase an errant pass, it's likely to be the setter. If your pre-beckon routine includes finding the setter, you'll always know, "she needs to be down"/"she can be up".
If you know the setter (and the opposite), it makes it possible to for you to make sense of the serve receive alignment. It's hard to explain in the context of a message board, but if you know the setter/opposite and place them on the floor, if the setter is back row, then you can identify the two players who go with them, because they are either the players on either side of the setter or the two players between the setter and opposite (either direction, depending on whether the setter is rightl/left back).
Clear as mud I'm sure, but that's what it is supposed to assist with.
