The primary purpose of a thesaurus is to help writers avoid repeating themselves when it comes to common words, a trait which can cause a book to feel underwritten and sloppy. Thesauruses provide access to synonyms for words both common and uncommon, allowing writers to diversify their vocabularies and expand their books' word choices.
Thesauruses can also help writers to brush up on their knowledge about parts of speech and to move forward securely using the right forms of words. Replacing an adjective with a noun is a surprisingly easy mistake to make, so certainty about parts of speech and their uses is valuable.
Thesauruses can provide access to more arcane and obscure synonyms and antonyms for common words. This can help a writer tailor the sound and feel of their language to another period of time or to another genre, imitating, for instance, the affected courtly speech popular in some fantasy novels.
While thesauruses contain synonyms, even these words are usually subtly different from one another, and those small distinctions should be considered carefully when choosing a replacement word. Minor and insignificant can be used interchangeably in many scenarios, but in others they suggest entirely different ways of assessment and are not appropriate replacements for one another.