The neck of Giraffes became elongated in an attempt to forage leaves on tall trees. this explanation was put forward by 

Lamarck a French naturalist, said that evolution of life forms had occurred but driven by use and disuse of organs. He gave the examples of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly, over the years, came to acquire long necks.