Certain species of birds will eat berries, others do not. Then,there are species who eat live prey, like robins eating worms. I live to the east of you in Ontario Canada, and my wife and I purchase whatever suet cakes are cheapest at a given time. The local flocks seem to like whatever we put out, as we regularly see 30-50 birds hanging around each time we refill the feeders.

TL/DR: I imagine most of the "variety" is more about marketing than anything else. The suet itself is the primary food source, anyway.