Williams v Roffey Brothers & Nicholls Ltd has been successfully applied in a variety of cases e.g.
•    Anangel Atlas Compania Naviera SA v Ishikawajima-Harima Heavy Industries Co Ltd:
•    Simon Container Machinery Ltd v Emba Machinery AB:
•    South Caribbean Trading v Trafigura Becheer BV:
•    Lee v GEC Plessey Communications:
•    Horwood v Land of Leather Ltd:
There is no valid consideration for the performance of an existing contractual obligation.
However, if a party has an existing contractual duty to do an act, this act cannot be used as consideration unless it confers a practical benefit or obviates a disbenefit.
Explain how each case shows either a practical benefit or obviates a disbenefit or both.