when should we consider a cdn for caching

So, I've been pondering caching lately, you know, for stuff like images and JavaScript files. We're all familiar with in-memory caches and Redis, right? But when should we actually consider using a Content Delivery Network, or CDN? What's the main stuff we need to think about when deciding? Is it just about how much traffic we're getting, or do things like where our users are located around the world, or the kind of content we have, make a CDN a better choice?