What engineers contribute before a gun fires

Why did the expedition not simply remove the guns from its ships and use them ashore?

Because engineers had to design the landing and movement arrangements and site the batteries; sailors could build and fire under direction, but guns alone were not a siege system.

Heavy artillery might require specially built systems simply to cross a beach and move inland. Engineers then selected the battery site and directed its construction, while sailors supplied labor and served the guns. Naval carpenters could execute woodwork and haul equipment, but they still needed specialists who understood the ground and shore-battery design. Available naval ordnance therefore required engineering support to become a functioning siege train.