I’ve got a fantasy world in which the landmass is essentially a large series of archipelagos and island chains. For a real world comparison, think the Hawaiian islands in range of size and distance from island to island. For their early history, these islanders travelled to and fro for commerce and war in boats of various sizes and types. Later, however, an empire emerged that linked the nearby islands with a series of titanic bridges.
For the purposes of the question, don’t worry about the possibility of building these bridges; magic is in place that makes the work possible, but not easy. The question remains, why would this empire choose to put massive work in to build these bridges when a rich shipbuilding culture exists? In case it is relevant, the empire in question has approximately Antonine Rome's technology, besides the magic, and is expanding in order to add strategic resources like iron and horses to its collection.