There benefit would be exactly what this top comment is about. For all those people in your contacts who don't use signal, you could still use signal to message them, it would just be an SMS message. It would therefore also become more enticing for people to switch from their stock SMS app to signal, because it would make no difference in terms of who you can send SMS messages to. It would allow for the signal user base to grow, solving the issue of "no one uses signal"