Further corrections to the ones already given above:
- Wallet security:
The longer the phrase the more secure your wallet would be. Preventing Intruders from guessing the phrase.
If you want more security for your seed phrase than 12 words, then the correct course of action is to generate a 24 word seed phrase, not to add an additional passphrase. Further, bitcoin private keys have 128 bits of security, so anything beyond a 12 word seed phrase doesn't add very much when considering brute force attacks.
- plausible deniability:
It completely denies knowledge of any particular attempt to guess the phrase words and claims such doesn't exist
Plausible deniability is the main reason to use passphrases, but just using one is not enough to give you plausible deniability. You have to keep the existence of the passphrase protected wallet secret, which includes ensuring there is no blockchain evidence linking it to your other wallets.
- Total privacy: This phrases aren't saved on electrum server and is only I have to you alone.
Your seed phrase isn't saved on Electrum servers either, and should also not be known by anyone other than you.