In the guide to responsive images in Drupal, you set up image styles, a responsive image style with sizes and a media view mode, and you never had to define a single breakpoint. That setup covers most images on most sites.
This article is for the next step: knowing where your layout changes, writing those widths down in a way Drupal understands, and using them for the one job where breakpoints really matter in the Responsive Image module, showing a different crop of an image on different screens. That's called art direction.
