Creating read replicas allows the primary database to offload read requests to the replicas, thereby improving the performance of the primary instance for write operations and providing faster read responses from the replicas. The deployment of read replicas in different Availability Zones (AZs) also increases high availability. The synchronization of read replicas is asynchronous, which helps in scaling-out read operations without impacting the write latency of the primary database. Increasing the instance size could improve performance but would not address the scalability or high availability. Multi-AZ deployment is for high availability and failover support, not for scaling read operations. Using a caching layer such as Amazon ElastiCache can indeed reduce database load, but it does not replace the need for read replicas when the goal is to scale read operations.