Because sometimes you need to make a molecule bigger before you can build something new—and then throw the extra pieces away. Ethyl N-(diphenylmethylene)glycinate is a protected form of glycine often known as O'Donnell's reagent.
Its bulky diphenylmethylene group isn't meant to stay. It's temporary molecular scaffolding. Under basic conditions, the carbon corresponding to glycine's alpha carbon becomes nucleophilic. Now chemists can attach a new carbon group.
Global Chemical Database