Why were the discoveries of Don Johanson and Tim White so important?

Assuming you mean jointly, the two worked together on the Lucy fossil, or Australopithicus afarensis. Lucy is important for a number of reasons. First, she was one of the oldest hominid fossils found at the time. Second, she was clearly bipedal while still having a small brain. Previous theories assumed that bipedalism would be foreshadowed or at least accompanied by larger cranial capacity, which was now disproved. The two didn't work together long and eventually had a falling out.