For automated backups, I/O activity is suspended on your primary DB since backups are not taken from standby DB

Amazon RDS automatically initiates a failover to the standby, in case primary database fails for any reason

To enhance read scalability, a Multi-AZ standby instance can be used to serve read requests

RDS applies OS updates by performing maintenance on the standby, then promoting the standby to primary and finally performing maintenance on the old primary, which becomes the new standby

Updates to your DB Instance are asynchronously replicated across the Availability Zone to the standby in order to keep both in sync