Why do you even need a neck that long, come on.

Answer 1: Because they were the dino dandies.

Answer 2…

Who Needs Necks? | AMNH

Some experts think the huge plant eaters may sometimes have stood on the edges of lakes and swamps, peaceably grazing on vegetation growing in shallow water. If they did, their long necks were one way to keep huge bodies on higher–and firmer–ground.