In addition to what @ruff said, in many countries, SMS is a “free” (= unlimited included in the monthly fee) tool that is widely used by the population. Maybe you don’t feel it because in your country, it is technologically outdated in favour of messaging applications.
Signal encrypt you text content, then forward your message to their server, and sent it to your recipient. Nobody knows today if Signal has the ability to decrypt your message (their server part is closed-source). No one can say what they do with your unique phone number that’s supposed to identify you either.
Where Silence is very strong, it does exactly like Signal, but with the advantage that the content is unreadable on my operator’s server. I was able to verify it (the phone operator offered a service to register SMS in a webmail) : my recipient and I could read our messages in Silence, while my operator’s server displayed an unreadable encrypted message.
Some people, included me, use Signal or Telegram but don’t trust them. We use them because they are the only few alternatives to Whatsapp (and it is already difficult to convince someone to discuss with you with those alternatives). XMPP is the same problem : you have to chose a server that you don’t know. And if you want to setup your own, it is difficult to do it for a standard person like me.
I am a bit disappointed about this SMS aspect of the Librem 5 : they promote privacy and give you the ability to use SMS, but it is like they don’t want to use this open-source Text-Secure protocol to secure SMS while it is obvious that it needs to be secured.