Why would a production app route different requests to different LLM providers instead of picking just one?

Different models have different cost/latency/quality trade-offs — a cheap, fast model might be fine for simple classification while a more expensive frontier model is reserved for complex reasoning, and having a fallback provider protects against one provider's outage taking down your whole AI feature. Spring AI's abstraction over `ChatModel` makes this routing swap relatively mechanical rather than a rewrite.