The new Courier client-side SDK for JavaScript provides an alternative to server-side SDKs, allowing developers to integrate Courier's functionality directly into their web application's front end, streamlining the process and catering to a broader range of scenarios. The SDK includes three new API calls: the identify call for tying a user to actions they perform, and the subscribe and unsubscribe calls for managing communication preferences with ease. This simplifies interactions, reduces workload, saves time, and accelerates implementation of new features, making it an invaluable tool in communication management.