Why ACLs instead of changing ownership?
When a web server runs as www-data and the dev squad needs to touch files in /var/www/html, the quick fix is usually chgrp the directory and slap a write bit on the group. That swaps the group ownership, which can trip up services that expect the original group or automated deploy scripts that hard‑code the owner. ACLs let you hand out the right permissions without touching the classic ownership columns.