You can reduce the load on your source DB instance by routing read queries from your applications to the read replica. Read replicas allow you to elastically scale out beyond the capacity constraints of a single DB instance for read-heavy database workloads.

To further maximize read performance, Amazon RDS for MySQL allows you to add table indexes directly to Read Replicas, without those indexes being present on the master.

Because read replicas can be promoted to master status, they are useful as part of a sharding implementation. To shard your database, add a read replica and promote it to master status, then, from each of the resulting DB Instances, delete the data that belongs to the other shard.

Because the Amazon RDS for MySQL engine also allows you to perform table-wide actions like adding indexes or new columns to read replicas, you can use its replica-promotion capability to minimize the impact of these actions. You would direct the DDL for the action to a read replica, promote that read replica to master status, and then redirect database traffic to the new master.