I see now . It makes sense if you have a predetermined listening spot (e.g. when adding a subwoofer).The idea is the speaker goes at the listening position and you play something like pink noise and then move the microphone around possible locations for the speaker to look for an even bass response. The speaker then goes in this position. As I said it's a common concept for subwoofer placement although usually done by ear. The idea behind it is tied to room mode behaviour and an assumed reciprocal behaviour such that what you hear at the speaker position when the speaker is at the listening position mimics what is heard for the reverse arrangement.
Thinking about it I wonder what REW's room simulator shows if this is tried. I'll have a look some time.
I prefer to optimise the location of both speakers and listening spot.