The main purpose is for support. Per the website:
As a GraphQL::Pro customer, you get direct access to the GraphQL Ruby gem creator and maintainer. Get prioritized support for issues and requests.
Note that we cannot use the Pro version directly in our product, since we are an Open Core product - we cannot require customers to purchase the Pro version, nor can we ship it.
Details on the billing account and gem licensing can be found in the Engineering 1Password vault.