Because you want to store them on your phone encrypted, so that no one else can read them without knowing your passphrase? Sure, the fact they're transmitted t

Because you want to store them on your phone encrypted, so that no one else can read them without knowing your passphrase? Sure, the fact they're transmitted to your carrier in a 100% readable form removes most of the benefit of this, but it can still stop prying eyes from reading your texts. Also, it's trying to be like iMessage. You have a single messenger, where you default to unsecure SMS for those contacts that don't have Signal, but default to secure messaging for those that do, all in the one app without you having to think about it.