Read Replicas: So, When Are They Useful?

Last week, we talked about the situations when a SQL read replica makes things harder than they have to be. This week, I want to talk a little bit about the math behind read replicas and scaling for throughput, and then talk a little bit about the circumstances in which a read replica is actually very necessary.When do you need to scale a database plan? When do you actually need to scale up a plan from, say, 4xlarge to 8xlarge?.