ecryptfs-add-passphrase hashing passphrase

Looking at the source for ecryptfs-add-passphrase, it is generating a signature for the passphrase by running SHA-512 a large number of times starting from the passphrase.

This signature is then passed to the kernel key ring as the "description" parameter, and is also passed into the mount command. From the description of this parameter it's just a unique name, it doesn't have any security value.

So why not just generate a random value for this description? Making it a derivative of the passphrase just sounds like it's adding another attack vector, however unlikely. And hashing it can take a significant time on an low power platform. Is there something else going on here?