I found this out as well during deployments, and my first answer is, no good reason at all!
"In their normal course of operation, RiOS-based appliances generate and store a collection of cryptographic secrets, authentication tokens, encryption keys, and certificates. This information is stored on-disk in file called the secure vault."
It stands to reason that on a first deployment their are no secrets to secure, so wiping the secure vault and starting from fresh is no issue. As to why this needs to be done, pass.