If you tap on the screen of your smartphone a little stronger than usual, a preview of the dialog box (chat/channel) will open. It is very convenient to check the contents of the conversation without opening the message. In addition, while holding the message preview, you may notice there's the notifications setting to “mute/unmute”, as well as an option to pin the message.
Although this feature exists in the usual iOS Telegram, it is visually implemented a bit differently.
If you've never noticed this little feature then you should try it. It's these subtle details that add to the overall convenience when using Telegram.