They want the exposure to aid in recruiting, fan support, fund raising and other aspects to enable them to get into the "stronger" half of the conference. For the B1G Illinois, Iowa and Northwestern fall into that category. BYU, with its ESPN deal and with BYUtv have the ability to grant exposure to anyone. By scheduling NIU, BYU is giving exposure to a local "weaker" conference opponent rather than the "weaker sisters" of the B1G. More exposure equals potential for greater 'popularity' locally.
SV Coug