Keep in mind that if you "add" passphrase you actually create a completely new wallet, with new addresses. It's like you'd have aone more wallet, with seed of 25+ words.
Why?
* Some can use that for extra safety by saving the seed in one place and the passphrase somewhere else.
* I've read that you can use this for plausible deniability. Keep a smaller amount in your normal wallet, keep the rest in the "special" one (but imho it's risky, you can too easy misplace your passphrase)

Keep in mind that if you "add" passphrase you actually create a completely new wallet, with new addresses. It's like you'd have a wallet with seed of 25+ words.
Why?
* Some can use that for extra safety by saving the seed in one place and the passphrase somewhere else.
* I've read that you can use this for plausible deniability. Keep a smaller amount in your normal wallet, keep the rest in the "special" one (but imho it's risky, you can too easy misplace your passphrase)