You spot a great art exhibition. Your close mates aren't into art. So you don't send it to anyone. Not because you don't know anyone who'd be interested - you do. But those people aren't the people you'd normally message. You'd have to think about who they are, find the right way to reach them, and make it into a thing. But the event is only a maybe - you're not going to do all that work for something you might not even go to.
Your interests don't map neatly onto the people you talk to most. You've got a few people who'd be up for gigs, different people for food stuff, someone who's into art, a couple who'd go to talks. But they're scattered across different parts of your life.
So you default to your main group chat, where half the stuff doesn't fit. You're not going to post an art exhibition in the pub WhatsApp. And you're not going to individually message three people you vaguely know who'd be into it. So the event just dies.
A CouldWe group organised around a shared interest changes this. A loose collection of people who are all into the same kind of thing. You save an art event, the people who care see it. No awkward message to the wrong chat. No guessing who might be interested. The right events just end up in front of the right people.