Thesaurus is necessary for effective information retrieval. A major purpose of a thesaurus is to match the terms brought to the system by an enquirer with the terms used by the indexer.
Whenever there are alternative names for a type of item, we have to choose one to use for indexing, and provide an entry under each of the others saying what the preferred term is. The goal of the thesaurus, and the index which is built by allocating thesaurus terms to objects, is to provide useful access points by which that record can be retrieved.
For example, if we index all full-length ladies' garments as dresses, then someone who searches for frocks must be told that they should look for dresses instead.