What are the rationales for token in js-based APIs

A lot of "javascript API" such as the ones available from google for tracking, etc... provide you with a token to initialize the API. What are the rationales for such a design 开发者_运维技巧? Is it to simplify tracking on the backend side for the service which provides such APIs ? The idea "sounds" natural to me, but being relatively new to web development, I don't think I really understand the implication.

Tracking and control. They can make sure you are only using it on registered domains and if usage stops conforming to their constraints they know who to contact and/or shutdown.