I can provide some answers. Telegram is my primary chat app.
- It offers you to simply share an alphanumeric handle and you can connect with anyone in the world. I can do a voice chat or normal chat really quickly and easily. It is the only famous truly Instant Messenger there is. And I can do it without sharing my name (unlike FB), email address, or phone number (unlike WApp).
- It is the only app where I add any woman to any group outside of family or other close-knit groups. When creepy dudes get women's cell numbers, it sometimes becomes very difficult for the women to handle. In Telegram, you can be part of large or large-ish groups without the women having to share phone numbers or real names. Blocking is really easy and tension-free.
- The video quality in chats is really high. In Android, only Google Duo came close.
- ONLY chat app in market with an excellent desktop app for Linux. Also Windows.
- I love their non-SJW-everything approach to allowed stickers. Emojis can only be PG-13 and "wholesome" (like banning gun emoji and replacing it with water gun- where I live no one owns guns - and guns are irrelevant to my politics and everyone else's). Telegram's stickers are really fun and creative.
- UI/UX is excellent. Really love chat themes and how I can edit them. Many components are customizable. Chat bubble colors, radii of rounded corners, etc.
- I like the fine-grained control over notifications from different chats (groups and persons).
- I like the lack of E2EE. I just like logging in and having access to all my chats. I don't have to be with the same device to be on the same chat.
- The simple yet effective image editor is nice. So is the text formatting with no fuss.
- I often use the auto image resize feature to downscale images.
- It is so seamless. It has replaced email when I want to transfer small files to my own devices.
- I am a selectively social person. It's good that even with 700 mn people, not many people are in it. The people with whom I don't want to interact more aren’t yet in Telegram. That's an appeal to me.
- Developing bots is a bliss. So easy and effective.
- SO helped a bunch of HS kids with their Bio study before a big test, the quiz feature was great. SO is not a technical person, and it was easy for her to set up a group, no phone numbers, create quizzes and so on. She loved it.