Query expansion and reranking are both valuable and things that even small LLMs are pretty good at. People are broadly not great at writing search queries, so a system that takes a poorly phrased question, turns it into several different actual search queries and then merges and sorts the results by relevance can have a big impact. All the better if the AI can ask the user some clarifying questions.

The issue is the opposite of hallucinations where the LLM ignores relevant information.

Sure you can combine this with a regular search but there’s opportunity cost whenever you say LLM + something else rather than just the other thing.