It's pretty popular in Europe. It is encrypted, you don't need to share your phone number like with WhatsApp.
I have no Idea, what LINE offers in terms of group chats, but afaik the group size in WhatsApp is limited, I n telegram it's not or the limit is huge enough to practically not matter. Telegram also apparently allows for easy integration of bots in channels.
If you only want to use it for private messages, there is probably no real reason to switch to telegram. If you are part of a community that uses it, it's obviously different. If you compare it with WhatsApp, I don't think WhatsApp wins in any other category than having been there first.