Application level checksums aren't for when the hard drive fails -- the OS will catch that, as every single operating system is already assuring hard drive blocks.

Application level checksums are for memory, driver, subsystem faults. And many enterprise database systems still do it by default, regardless of how redundant it might seem. Even though your NAS, file system, OS, and even the individual drives are computing it too, systems like SQL Server, Oracle, DB2 and others default to calculating and verifying checksums on all pages.