I wish somebody more knowledgeable would give an answer, but here's a short version of how I understand Malta, based on reading a number of books both about the African campaign and the global war strategy.

The Allies won Africa in large because they were able to interdict Axis supply lines to Tunis. Without Malta, this interdiction would have been nearly impossible as airplanes did not have the range to get to the shipping lanes from other Allied bases at the time. If the Axis had taken Malta, their control of the sea and air in the central Mediterranean would have required much more men and material to dent.