Here in europe every club/association/group has a whatsapp group chat. For instance here since the official app provided by the government has a super clunky UX most people get information from primary school through a whatsapp group chat managed by the parent's representative who has exclusive access to teaching group.
As a counter to your question I've never used whatsapp and never saw a reason. What group chats? Are they groups of personal friends or mostly things you would 'follow' like a football club?
Additionally whenever there is a social event a group chat is created so that people can discuss the organization, and after the event share their sentiments, pictures, videos.
It is never mandatory to participates in all those group chats but a lot of info go through them and they are usually useful. And the family group chats are great when you only get the chance to meet them more than a couple times a year.
Glad to hear they aren't mandatory, that would be my fear for certain things.