Why would a researcher use a secondary source instead of a primary source when analyzing a historical event?

During the cold war, the united states was involved in certain regional conflicts due to fears later called the domino theory, which stated which concept?

Me i really need which of these was an accomplishment of the han dynasty? the invention of the seismograph finding a water passage to europe the invention of the ballpoint pen the invention of the abacus

Why did kennedy choose lyndon b. johnson to be his running mate