Headless architecture is a response to the way content has evolved: for a long time, most web content was maintained for and delivered through a browser, often as a web page. Editors structured their content with layout in mind.
But new channels/applications continue to emerge. For example, users consume content through new interfaces with different form factors such as smartphones/tablets, voice assistants, internet-of-things devices, chat bots, or wearables.
Headless CMS architecture is the foundation for addressing these new challenges. With such a CMS, content can be easily managed in one place and delivered to multiple different locations by decoupling the content structure and rendered layout.
Editors now are forced to think in semantic content structure (e.g. headlines or emphasized content) which then can be consumed and used in the different channels instead of tailoring the content to match a specific layout (e.g. line/word wrap or bold/colorful text). The respective channels are responsible for rendering and displaying the semantic structure in their native interfaces.