Modern side by side shotguns have a device that causes the internal hammers to be cocked and the springs set when the action is opened. Also within the action are the sears and firing pins.
With traditional exposed hammer shotguns, the hammers and firing pins are on the exterior. Cocking the hammer exposes the firing pin end, pull the trigger, the hammer falls, and drives the firing pin into the shell's primer. Unlike the modern gun, opening the action doesn't re-cock the hammer spring, it must be done manually.
In an attempt to recreate the old style guns, modern guns are modified to remove the automatic cocking device, but the firing pins and hammers remain internal. External cocking levers are shaped to resemble the old type of hammer and are operated in a similar way.
The benefit of this design is it allows the mfg to make an old looking gun with a minimum of new parts, and the firing pins don't have to be relocated. For the shooter, having the external cocker, means opening the action doesn't required the energy to operate an internal cocking mechanism, so it's faster and easier; but, any saved time and energy must be expended operating the external cocker.