A brain wallet is a good choice for someone who doesn't want to write down or record a password in any way. Everything you need is memorized and stored in your brain. This provides added security provided you can remember your passphrase with 100% reliability.
When you create a brain wallet, the NanoWallet will create a private key which is derived directly from your passphrase. As the name suggests, a brainwallet is meant to be only secured by a password/passphrase which can be remembered without further backups. You can log into a brain wallet from any NanoWallet on any computer; a .wlt file is NOT needed, nor is the private key ever stored in the wallet file as it will be derived each time independently.
Because of that reason, we recommend an extra long passphrase, e.g., at least 12 random words.