First, you can negotiate a larger TV contract, depending on who you add. Means a bit more money, and since conferences share bowl revenue, you might actually have 1 or 2 more teams going to bowls. You have more TV markets and a wider diversity of interest (see the TV contract part). Having more teams also increases the probability that the conference will be home to more "good teams," which increases exposure. It also increases the difficulty of the conference schedule, which ups the credibility of top conference teams when at large teams are being selected for NY6 bowls or CFP spots.

With this overall improvement, it probably makes it a lot less likely that teams will go through the hassle (and it sounds like enough of a hassle) to jump ship for greener pastures since those pastures are likely now only marginally greener (instead of WAY greener like they seem now).

Also, you show your universities that they can trust you as a conference to try and keep them together and look out for their interests. With the way the B12 has been managed (by not adding teams soon enough, etc.) I might consider taking the offer from P12 or SEC if they offer me (if I'm a B12 school).