The Direct API does very much the same as the REST API, with the main difference being that the render parameters are all passed in the URL, and you'll receive the rendered result back as part of the same request. As such, you can replace any image or video on your website or emails with a render generated by the Direct API.
The Direct API can render for up to 100 seconds. In most cases, this will be sufficient processing time. Use the REST API for long and heavy videos.