Almost two years ago, this was how I explained why you might want to use Signal:
Sometimes you need to send a message that you can be certain will stay private. Perhaps a friend urgently needs a place to stay while you’re out of town, and you must give them your alarm code (and maybe tell the neighbor who has your spare key how to recognize them). Or perhaps you want to discuss a medical condition, or something that’s perfectly legal but might still get you into trouble at work, such as whistleblowing or staging a protest.
it's been nearly 2 years since I wrote this (free to read) piece about Signal: why you want it, how it works, why it's better than most alternatives. I even included an example of how little information Signal can hand over if it gets a warrant for a phone number law enforcement already has
I did a followup with tips for using Signal on Windows (just as secure, as long as you don't go through a service that logs it, let press photograph your screen or invite the wrong people to group chat), covering screensharing, usernames, why there's no web app and how to use it without a smartphone
the second piece needs an @askwoody.bsky.social subscription; that's a pay-what-you-want deal (minimum $6) and with that you can also see the forums where I answer questions about why there isn't an officially supported Android tablet version and how to use the hardened Molly fork instead
Signal
WhatsApp
E2EE
secure messaging
encryption
privacy