They are both used to improve the quality of a document. The Spell checker is used to correct obvious mistakes in the spelling of words, and the Thesaurus is used to correct a more subtle error, that of using the wrong word. For example, you might use the phrase in your document, "She was a good person." And, after looking over the sentence, realize that the word good doesn't really express the way you feel about this person. Looking up the word good in the Thesaurus you find the synonyms benevolent, honorable, and competent. After thinking again about how you really feel about her you decide honorable describes your feelings better than good.