When a CDN cache isnt enough, when do you need to add another caching layer?

I'm on a project with users all over the world, and right now, we're using a CDN to store static stuff and some API responses. It's doing a good job making things faster for people far away from our main servers. But I'm starting to think, what if the CDN isn't enough in some situations? Like, what about dynamic content that changes all the time and is accessed a lot? Or data that needs to be super consistent across different regions, more than a CDN can really handle? So, are there certain kinds of data or ways people access things where you'd absolutely say, "Yep, you need another caching layer, even with a CDN?" Maybe for internal communication between microservices? I'd love to hear about real-life examples where a CDN just wasn't enough, and you had to bring in something like Redis.