Information architecture is like the frame and foundation of a building. It's not a building by itself, but you can't add the frame and foundation after the building is up. They're critical parts of the building that affect the whole of it. Buildings without frames do not exist.
It's hard to relay your intended meaning through façade alone. When your structure and intent don't line up, things fall apart.
Imagine trying to open a fancy restaurant in an old Pizza Hut. The shape of its former self persists in the structure. The mid-nineties nostalgia for that brand is in its bones. Paint the roof; change the signage; blow out the inside; it doesn't matter. The building insists, "I used to be a Pizza Hut."
(Now type "used to be Pizza Hut" into Google's image search and enjoy the laugh riot!)