It’s enabled by default, so that you can use a hidden wallet if you want.
It also provides plausible deniability. If someone asks you for your passphrase, you can say “I don’t use that stuff, it’s enabled by default, but all my funds are in the normal wallet”……. regardless of whether you actually use passphrase or not.
There is no advantage to disabling it and the fact that it is even a setting is a historical artifact.
Not 100% sure, but I think you always have a passphrase. What happens when you click normal wallet is that you type in an empty password. So if you use another software with trezor, they will ask for a password and you can just type in nothing to access your funds