only to remove it later? Protection. 4-Benzyloxyaniline hydrochloride offers a neat example. Its synthesis can begin with 4-nitrophenol. First, the phenolic OH is converted into a benzyl ether: O-H becomes O-CH2-C6H5.
The oxygen is still there. But now it's wearing a temporary chemical shield. Next, the nitro group can be reduced to an amine while the protected oxygen remains intact. Finally, hydrochloric acid converts the amine into its hydrochloride salt.
Global Chemical Database