The evolution of the extremely elongated neck of giraffes, which can be up to 2.4m in length, has been the subject of intense debate since the time of Darwin.

The 'competing browsers hypothesis' was originally suggested by Charles Darwin and challenged only recently. It suggests that competitive pressure from smaller browsers, such as kudu, steenbok and impala, encouraged the elongation of the neck, as it enabled giraffes to reach food that competitors could not."

Source : Wikipedia, link below

However :

A 2010 study found that adult giraffes with longer necks actually suffered higher mortality rates under drought conditions than their shorter-necked counterparts. This study suggests that maintaining a longer neck requires more nutrients, which puts longer-necked giraffes at risk during a food shortage."

Source : Wikipedia, link below

Another, more recent theory, suggests that the long necks evolved because of the 'necking' contests (in which males use their heads as weapons) to establish dominance and obtain access to sexually receptive females.

See: Wikipedia

A third, more recent hypothesis is that the long neck gives the animal enhanced vigilance to avoid predators :

Giraffe (Giraffa camelopardalis), with their long neck and legs, are unique amongst mammals. How these features evolved is a matter of conjecture. The two leading ideas are the high browse and the sexual-selection hypotheses. While both explain many of the characteristics and the behaviour of giraffe, neither is fully supported by the available evidence."

Source : Biology, 5(3), 35;