Note that the "additional phrases" and the nonce is an optional values, which means that you could only generate the same seed phrases with the same email and password.
Which would be highly insecure.
Backup option is provided in case you forgot your email or password, or any other values you have used and it is recommended if you tend to store valuable crypto.
Which is entirely my point. Given that you
should be backing up your wallet recovery process offline, then why not generate your seed phrase securely and back that up, than risk generating it insecurely by using this method and then backing up your additional phrases and so on (which also don't have a checksum or any way to identify an error).
Of course, you could generate random phrases via the website which is generated by the browser cryptography API, and make an encrypted backup of it
https://bip39.org/#mnemonic.
I wouldn't recommend that anyone uses a website based javascript generator:
https://bitcointalk.org/index.php?topic=5324030.msg56590276#msg56590276