In my experience: Only for drug dealers, cheaters, people doing sex stuff secretly for whatever reason (e.g. not being out of the closet), "underground/alternative vibe" communication channels (e.g. raves, hackers) and russophiles...
Like you said "in my experience."
You could use it for other things lol. My entire boring friend group uses it for none of those.
This sort of anecdotal evidence tells more about you than Telegram ;)
It's a standard chat application, people are also using Facebook, WhatsApp, etc for illegal/immoral discussions. I've been using it as my main chat client for more than 10y and have never used or felt the need to use the community channels, it's just a great messenger.
Doing illegal stuff in public Telegram channels is extremely stupid, they aren't encrypted, you're pretty much giving your information to law enforcement (I know Telegram is often blocking sharing info on their users to law enforcement, but you can be sure agencies are monitoring your telegram channel discussions).
The Russophile comment is pretty frustrating given Telegram is the most commonly used messenger in Ukraine and lots of Eastern Europe...