The corporation as a whole picks up those functions which cannot be provided by business units or service centers. For these functions, it's either difficult to make billing a meaningful incentive to use the service (as with finance and accounting--it's not clear how to meet legal requirements for accounting if every business unit contracts an outside accounting firm), or so dangerous to the company if business units don't use the service that there's a need to eliminate all disincentives for using it (as with legal, as discussed later).
The number of corporate functions should be minimized. This is where bureaucracy grows, since the incentives for keeping lean and mean are so weak. If someone comes up with an interesting idea for how to turn a corporate function meaningfully into a service center, we should go for it.