Cetaceans traded neck mobility for streamlining. In most species several of the seven cervical vertebrae are fused, locking the skull to the body so that the animal turns by banking its whole trunk. The beluga retains seven unfused cervical vertebrae, and can consequently turn its head laterally and dorsally to a marked degree.
The likely advantage is manoeuvring in cluttered water. Belugas forage in shallow estuaries, among river sediment and beneath sea ice, where prey is close and cover is complex. The same flexibility extends to the melon, the fatty forehead lens used to shape echolocation beams, which belugas can visibly deform, flattening and bulging it.
The rest of the body is not similarly loose. Belugas have no dorsal fin, replaced by a tough ridge that suits swimming under ice, and the head mobility is a matter of degrees rather than the sweeping turns often described in popular accounts.