What does it do?
Assets such as an image, CSS file, or JS file are saved with an expiration time, or lifetime, when they are saved on the CDN's cache. After the lifetime expires, the CDN fetches the asset again from the origin server the next time a user requests the asset, and the CDN sets a new lifetime based on the caching headers prescribed by your origin server or what's configured under the "Lifetime" dropdown.
Enabling "Content Persistence" allows you to set an expiration time past the content's configured lifetime. This is particularly useful if your origin server is unavailable. The CDN will keep delivering these assets to your customers.
When and how should I use it?
This feature is useful for unstable origins and sites with content that's not updated on a regular basis.
When this feature is enabled, the CDN will try to fetch a new version of an asset after it's expired. If the origin does not respond, the CDN delivers the cached expired asset to ensure stable performance.