LLMs are a kind of thesaurus, in a way - but instead of just words, they also handle phrases, sentences and whole paragraphs. The conversational interface is just a gimmick on top.

Also, thesaurus isn't a good tool for exploring an unknown problem domain. It gives synonyms, not related terms. LLMs let you input a layman description of your problem, and get an answer that's using correct domain terms and phrases (even if using them incorrectly).

I imagine Google will add such a function. They probably tried already - I've heard that current search is already powered by ML models to a degree.