One reason to have external hammers.

Recoil.

Double discharges.

Here is David shooting his .600 Nitro Express double. A Holland & Holland with external hammers.

David quite wisely only cocks one hammer at a time. If the second barrel went off as well it would be quite dramatic. Either from the lock slipping or the shooter twanging the second trigger under recoil. And the latter happens more on safaris than many a client ever will admit per PHs.

David under recoil. Love these images.