just so they can remove it later. That isn't wasted chemistry. It's protection. Chloromethyl methyl sulfide is a tiny two-carbon reagent used to install the methylthiomethyl, or MTM, protecting group. Suppose a molecule contains an alcohol.
The -OH group is useful—but perhaps not yet. During the next few synthetic steps, it might react when the chemist wants something else to react instead. So the hydroxyl group gets a temporary molecular cover: -OH becomes -O-CH2-S-CH3.
Global Chemical Database