Not just team building, but negotiating with real intelligence for trades, planning your draft knowing that your competition is able to find the gems just like you are (this really changes things, trust me), and then of course human intellect in game versus Madden AI. A human being is not going to play you press twice in a row after you burn him for a touchdown. A human being is not going to give up the corner route every time you come out in a tight formation. The CPU AI? Not so much.
Now, you need people committed to NFL SIM play in order for it to be fun, but things are definitely far more interesting with human competition.
There are downsides to online franchise, such as occasional lag and scheduling conflicts, but the game feels much more robust when you're not playing against idiotic CPU AI.