Pitt adds a two things.
1) A major sports market
2) Success in both major sports.
Dividing the conference into divisions also makes sense so you don't get wildly uneven scheduling (Remember when the Big 10 champ never played Ohio State or Michigan a few years back?) and you get to foster tighter rivalries. You get the benefits of a small conference, but the prestiege of the big ones. Perfect setup.
Pitt allows the Big 10 to add some depth. Too many bad programs in each sport. Northwestern, Pudue, Indiana, Illinois, Michigan State and Minnesotta are just bad football programs. They may have a standout season here and there, but they've been consistantly bad for too long.
That's an 11 team conference with 6 bad teams (on a year to year basis). Let alone now that Michigan is down. That leaves Ohio State and Penn State. Iowa and Wisconsin are good programs, but they aren't annual contenders.
The Big 10 is hoping that Pitt can slide into the Iowa/Wisconsin grouping of good progams. Win between 8-10 games a year, win some bowl games and threaten for the conference a couple times a decade. Obviously they wanted Notre Dame, but the Golden Domers have no interest in joining the conference.
Unfortunately, I don't really see Pitt as that program. Ohio State and Michigan already steal recruits from Pennsylvania...and Penn State is already the big brother. It's only going to get worse once the difference is played out on the field, yearly. Pitt should compete initially because they still have some talent and they can use this new Big 10 status for a few years. Eventually though, the annual losses to Penn State, Ohio State and Michigan (plus having to deal with Wisconsin and Iowa) are going to take a toll.
Pitt fans....Michigan State football is your future.