Except you didn't? You paid for all-you-can-eat-on-a-phone, and 5 GB extra. So it's a dormitory with unlimited dining hall, but you can only take one plate a week outside of the dining hall.

For the provider, the point of this design is that there's only a certain amount that people are willing to download on a phone or eat in a dining hall. For the consumer, they don't have to count their data or their servings as long as they follow a simple rule - they get predictable billing.