We don’t use Signal for SMS, we use Signal for E2E encryption. Signal for SMS is useless.
Of course you don’t because it doesn’t exist anymore. When it did, it was popular for exactly the same reason as iMessage: if the person you’re messaging also has Signal, they get automatically upgraded to private and secure communication, if they don’t it just gets sent as SMS. In this way you don’t have to bounce back and forth between apps to figure out which one your recipient is using.
Of course you don’t because it doesn’t exist anymore. When it did, it was popular for exactly the same reason as iMessage: if the person you’re messaging also has Signal, they get automatically upgraded to private and secure communication, if they don’t it just gets sent as SMS. In this way you don’t have to bounce back and forth between apps to figure out which one your recipient is using.