Why use your application-level cache if database already provides caching?

Modern database provide caching support. Most of the ORM frameworks cache retrieved data too. Why this duplication is necessary?

Because to get the data from the database's cache, you still have to:

1. Generate the SQL from the ORM's "native" query format
2. Do a network round-trip to the database server
3. Parse the SQL
4. Fetch the data from the cache
5. Serialise the data to the database's over-the-wire format
6. Deserialize the data into the database client library's format
7. Convert the database client librarie's format into language-level objects (i.e. a collection of whatevers)

By caching at the application level, you don't have to do any of that. Typically, it's a simple lookup of an in-memory hashtable. Sometimes (if caching with memcache) there's still a network round-trip, but all of the other stuff no longer happens.