Why AWS RDS service needs two subnets from different AZs

RDS is built to be Multi-AZ by design, as the best practice for any production environment must use this approach. This will allow you to be sure that in case of an event in one AZ your database will be able to fail-over to the second one. I know that probably you are trying to deploy just one single RDS instance in a single AZ, but the system requires to configure two subnets, as maybe in the future you want to transform your RDS instance in a Multi-AZ instance, this is the reason why you need always two AZ. Defining Subnets is not a process that have any cost, some times the cost is related to IP address availability, but nothing else. So, i am afraid you will have to setup two subnets in two different AZ.

All this is explained in AWS documentation here: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_VPC.WorkingWithRDSInstanceinaVPC.html

best.

That seems to be the common pattern for some other resources. Let's say your RDS cluster contains 2 db instances, one for writer and the other for reader. For better availability, they better be located in different zones from each other.