- Messages send quickly and reliably, even under poor and sometimes hostile network conditions. Telegram just seems to work even when other chat apps struggle.

- Telegram uses usernames instead of phone numbers by default, which is good if you're using it as an IRC replacement instead of an SMS replacement.

- You can have the client open on essentially unlimited devices simultaneously, including a web app if you need it.

- Messages can be edited at any point after sending with no expiry.

- You can schedule messages to send later, or send a message silently so it doesn't wake people up.

- Different group types - announcement channels, Discord-style groups with sub-channels, flexible moderator roles, etc. (I believe WhatsApp has some of this.)

- Support for bots, which is also very helpful for managing large communities.

- Community-created, sharable stickers. Seriously, people underestimate how nice these are.

The downside is that a lot of this requires state to be stored on the Telegram servers, so most chat's aren't E2E encrypted. (They do have an option for E2E encrypted private 1:1 chats, but you lose most of the polish by using that.)

Also, the official apps are open source, so you can modify them if needed.

I'll add a few more:

- insanely fast search, chat history browsing and in app navigation - unlimited unencrypted cloud storage, your chats and docs always stays available - ability to send very large files - ability to host large video and voice chats - chat automation - auto translation and transcription - mini apps - open source client, with lots of customization - phone number less sign up (you can purchase a burner number from them and sign up with that, I guess it costs their crypto (ton) tho) - sending gifts