As far as I know, the only case when it matters is at airports that don't have separate lines for selectees, so that the WTMD person asks for the BP again. In that case, if TDC marks somebody a selecteee, they don't want the person to simply pull out a copy of their BP without that mark. I suspect the major reason is just that TDC having to actually do something with the BP makes them pay more attention to it.