Why were the smith and cuvier discoveries that similar groups of fossils in distant rock layers could be correlated such a major breakthrough in geology?

This was a major breakthrough in geology because it meant that sedimentary strata in diverse parts of a country or area could be correlated as belonging to the same formation which means they were formed at the same time historically so helped to put them in the correct sequence. If these strata contained valuable minerals like coal then it could aid in the search for coal. This helped William 'Strata' Smith in England to create his first geological map in the world of a whole country.