Why dest_addr persistence is needed when deploying BIG-IP with Oracle RAC DB?

Hi everybody.

I'm reading "Configuring the BIG-IP LTM v11 for Oracle Database and RAC" to loadbalance Oracle RAC Database.

This guide describes about default persistence(dest_addr) profile and fallback persistence(source_addr) profile.

K8367: Deploying F5 with the Oracle Application Server

https://support.f5.com/csp/article/K8367

I can understand the reason persistence(source_addr) is needed but I can't understand the reason persistence(dest_addr) is needed.

Please tell me why both dest_addr and source_addr persistences are needed for Oracle RAC DB.