The neck connected the head to the rest of the body.
Probic vent at the back of the neck of a Sontaran's armour. (TV: The Sontaran Stratagem
The most vulnerable part of a Sontaran's armour was the probic vent at the back of the neck. (TV: The Time Warrior, The Sontaran Stratagem)
Raxacoricofallapatorian criminals such as the Slitheen wore compression field collars around their necks. (TV: World War Three)
Sometimes the Weeping Angels would kill their victims by sneaking up behind them and breaking their necks. (TV: The Time of Angels / Flesh and Stone, The Lost)
Honey Vox owned a Treed Crystal, which she wore on a chain around her neck. (COMIC: Jewel of the Vile)
The last time the Doctor met the Krillitanes before they were bat-like they looked just like humans but with really long necks. (TV: School Reunion)
The Twelfth Doctor thought that Clara Oswald had a long neck. For that reason, he jokingly renamed her "Clara Ostrich". (PROSE: The Blood Cell)