The practical benefit shows up in everyday use. If your logo sits too close to the edge, the text on offer posts is too small, or the accent color needs to change: you change it in one place, and every post after that comes out of the system correctly.
The alternative would be to manually fix every single post in post creation. That costs time every week and leads to posts looking inconsistent with one another.
Important for networks and agencies: centrally maintained templates are also the way to keep a consistent look across multiple accounts. How managing multiple accounts works is explained on the agency mode page.
