It’s an old bit of conventional wisdom. When writing, never use a thesaurus. Good advice, for the most part, because a writer should never use a word that is not already known to her. That she doesn’t already use naturally in conversation and print. Pumping up writing with unfamiliar words comes across as stilted and unnatural. And the writer always runs the risk of using the word incorrectly, and appearing foolish. So, never use a thesaurus to find new words. However…
I use a thesaurus on a fairly regular basis, for two reasons. First, as a memory aid. I will want to write a word that I know very well. The word will lurk there, just on the edge of my consciousness, but won’t appear to me. Very frustrating!! But what will occur to me is a word that is very similar. I look up that word in a thesaurus, and, lo and behold, there is the word that I couldn’t quite remember. Very useful.
Second, as a means to find words that sound a certain way. A significant part of writing isn’t just using words that convey a given meaning, but using words that sound a certain way. That have a desired tone and cadence beyond the meaning. Some words simply sound better than others, either generally, or within the rhythm of a given phrase or sentence. A thesaurus provides synonyms, and thus an array of possible sounds and emphasized (or not) syllables. Again, I would never use a word with which I am not already comfortable, but a thesaurus presents a range of auditory options.
A final note. An amusing cautionary tale regarding using a word without quite understanding its meaning. In 2006 a movie called Factotum, starring Matt Dillon, appeared. At the beginning of the movie, beneath the title, is printed a definition of the word factotum. It reads something like this: “a man who holds many jobs.” (This isn’t the exact quotation, because I don’t feel like finding the movie, but it is close enough.) And what is the movie about? The main character, Dillon, can’t hold down a job, so he goes from one employer to another to another, and so on. Too funny. The writer didn’t realize that the word describes someone who fulfills many types of tasks within the same/one job, not someone who goes from job to job to job. I’m not sure how no one, not one single person attached to the movie, realized the mistake. But the movie serves as a great reminder not to get fancy with unfamiliar wordplay.