This is just something I have read so I cannot say if this is the reason but......From what I understand, folks ask this question so they won't get caught double-dipping. Like, if OP-A uses pharmacy-A and OP-B uses pharmacy-A as well then that person will go to another OP that uses pharmacy-b so they won't get called on it.
I don't know if that is the reason or if my explination is clear or not but that's what I have heard.

Hope this helps....