An API serves a read-heavy workload — roughly 90% reads, 10% writes — all hitting a single primary database. The primary is CPU-bound on read queries while writes are light. Scale read throughput without touching the write path.
Read replicas scale read throughput by fanning reads out across copies of the primary, each kept up to date by replication. Writes still go to the single primary. The trade-off is replication lag: replicas are eventually consistent, so reads can be slightly stale. This is distinct from a failover replica (which targets availability/HA via promotion) and from an OLAP split (which isolates heavy analytical queries).
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