In a retrieval pipeline, why add a reranker after the vector search stage?
Not quite.
Bi-encoder retrieval embeds the query and documents independently, which is fast but loses fine-grained interaction between their words. A cross-encoder reranker reads the query and each candidate together through one model, capturing exact-term and reasoning-level matches — far more accurate, far too slow to run on the whole corpus. So the pattern is retrieve top-100 cheaply, rerank to top-5 accurately: a classic recall-then-precision funnel.