The APIs.json created date reflects the first day when an APIs.json contract has been established to define the relationship between API producer and consumers. The created date provides a clear line in the sand for when an API contract was kicked off, and the clock was running to deliver the APIs needed to satisfy the contract that was in motion.
Having a created date will define the overall maturity of an API contract and the APIs it references. More time does not equal maturity, but depending on the scope of change over time, one can get a pretty good idea of what is happening (or not) with an API, or across many APIs. The created date property for a contract is one of several key properties of an API contract that will help manage API change.