A thesaurus lists synonyms or similar words while a dictionary lists the meanings or definitions of a word.
Sometimes a particular word may not be what you exactly mean, so it is better to look it up in a thesaurus to find another word that will describe what you exactly want to mean.
For example, the definition of the word
good as an adjective will mean 'of a favorable character or tendency'. You may want to say 'a good man', but to be more specific, you might want to describe him as 'a decent man' or 'a righteous man', etc. You can use the thesaurus to find a more definite word to describe the man. They all mean good, but it can be made more precise with your choice of the word to use.
Am not really an english expert. I merely want to use better words than the more 'generic' ones to be more accurate with what I want to convey.
BTW, vagina still isn't in the meriam webster thesaurus. Now that's intriguing...