A thesaurus is very helpful when you come across a new word and can't figure out what it means. Most of the time, it'd include both advanced and general (simple) words; hence, it makes it easier for us to get familiar with the meaning of the word. However, the question you're asking also includes "phrases", and since I don't think a thesaurus will give you synonyms of phrases, it is much more convenient to use the internet as it is faster and easier to use.
