I don't want them to have access to my messages and make a profile out of me, even less of billions of people. That's why I also care about metadata: social graph is valuable for profiling.

I don't write anything that would be interesting to the NSA, I'm not a target for them, so I don't care if they see I use Signal.

BTW, say you have an anonymous Telegram username, I'm pretty sure it's trivial to find your name from the content of your messages. Or just from the metadata.