That sounds pointless. If you have real crypto, tossing the key (in a way that it cannot be recovered) is absolutely enough.

It's not pointless - it's pretty standard practice.

For example, although drives in Google Cloud are encrypted at rest, once they are decommissioned the drives are physically destroyed: https://cloud.google.com/security/deletion/#ensuring_safe_an...

> When a hard drive is retired, authorized individuals verify that the disk is erased by overwriting the drive with zeros and performing a multi-step verification process to ensure the drive contains no data.

Thanks. I guess the reason is to avoid future decryption due to either a new crypto attack or to avoid being vulnerable to potential errors in key generation.

This is massive overkill. Once is enough. If the drive is already encrypted, just wipe the key.