The release of WordPress 4.7 gave REST API endpoints to 27% of internet. Now the data (posts, comments, taxonomies, terms, users, options, etc.) from within your CMS is available for interaction. In order to get data from your site you have to send a request. The burden is on your API client to pester your API server with requests to ensure they have up to date data.
We live in a crazy mixed-up world with bots, microservices, and IoT. Polling the same API endpoint to see if something changed is taxing and inefficient. There is a better way with Webhooks.
Learn how subscription endpoints are the key to a real-time REST API. Notify a client that a new post published. Post a message to a Slack channel when a new comment is awaiting moderation. Receive a text message when a user resets their password. The use-cases are endless.