When do you really, really need a CDN, and when is it just kind of a bonus?

We're trying to figure out the best way to get our content out there for a new service and thinking a lot about CDNs. I mean, if we're just sending static stuff to people all over the world, then yeah, a CDN is a total given. But what if the content is more dynamic, or if most of our users are in one specific area? When does a CDN stop being just a nice-to-have performance boost and become something you absolutely need? Are there certain ways people use our service, or types of content, where dealing with a CDN is always worth it, even when we're just starting small?