The EnableCachePersistence setting is one of the AppSettings
that is contained in the web.config file.
In versions 1.x-4.x, DotNetNuke's module caching model was fairly simple and defaulted to using memory for all cache storage (Note: since 5.2.0 DotNetNuke uses a provider model which allows the selection of how to cache entries for both module and page caching - the Professional version ship with additional page caching support).
The introduction of the EnableCachePersistence setting was to allow a simple way to globally change the cache storage from memory (RAM) to file-system based. This is predominantly done by web farm setups as they can share a common file system. Some sites with long-lived content may also utilise it to ensure that the cache persists across asp.net application recycles as the cache can be built directly from disk instead of from scratch through the database.