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Encrypting the drive ensures, presuming the key is otherwise lost, that any data recovery methods which might exist could only get an [apparent] stream of random bits

That said … there's really no legitimate reason to do this, because doing

dd if=/dev/zero of=/dev/<dev-in-question> 

is pretty damn good! (on UNIX/macOS/Linux/*BSD)

And if you're really worried, run

dd if=/dev/urandom of=/dev/<dev-in-question> 

after you've zeroed it (or before – either is fine)

If you're on Windows, it's not quite as simple – so encrypting then formatting is good enough (for anyone but a nationstate – and if they're your threat model, encrypt, format, encrypt, format (with a different filesystem))