It's for getting synonyms. If you find that you awkwardly keep using the same word in a poem, you would consult a thesaurus to find one of its variants. Or if you wanted to look clever you would consult a thesaurus to find a big word that means the same thing as a small one.

astralboy has found a thesaurus entry for one definition of the word "theory", but it doesn't provide synonyms for other less common definitions such as in "number theory" or "the theory of gravitation", which would be words like "corpus" and "scheme" respectively.