For many organizations, a headless CMS are going to be more and more of a requirement, as more content is delivered to non-web or and non-mobile contexts. Some examples:
1. Shipping content into voice or chat bots.
2. Shipping content to embedded devices like touch screens on home appliances or factory machines.
Going beyond the separation of the data store and the presentation layer: A lot of people haven't yet felt the pain of using traditional relational data for these nontraditional channels. In voice and chat, for example, a knowledge graph is much more helpful than a bunch of relational data (or at least alongside relational data).
On the flip side of this all: my wife runs a blog, and I doubt she'll ever need more than Wordpress.
So, as usually, pick the right tool for the problem.