I have repeatedly seen the claim that a thesaurus is a good way of “sounding smarter” or “more sophisticated”. My take is radically different: Anyone who does so, is doing both himself and his readers a disservice, leaving him with a text that is harder to read and may well mean something different from what he intended. (Notably, the matches in a thesaurus are often only approximate, or correct only with regard to one aspect of the original word: I recall one episode of the TV series “Friends” where the nit-witted Joey Tribiani used a thesaurus to write an impossible-to-understand letter signed “Baby-Kangaroo Tribiani”...)
From my POV, a thesaurus has the following legitimate uses:
Finding a word that is less “sophisticated” and/or more likely to be understood and accepted by the intended readers.
Finding a word that better matches the intended nuances of meaning than the original word.
Avoiding endless repetitions of one particular word by use of synonyms.
Outside of the area of writing, I would additionally consider a thesaurus a legitimate means of learning new words; however, I have some doubts that this use is a good idea—an encyclopedia or ordinary dictionary will often be better suited.