Usually you don't want the blocks to be shown in the same order on mobile devices as on desktop. For instance on the Drupal bootstrap theme, the left sidebar will be shown just below the menu on mobile devices, that is not always what you want or would expect.
https://stevepolito.design/blog/move-block-different-region-mobile-layo…
Read more about breakpoints: https://www.drupal.org/docs/8/theming-drupal-8/working-with-breakpoints…