anyone knows why the Android developers team have implemented the Content-Resolver as a middleman to get data from a Content-Provider. Why not asking data directly from the Content-Provider ? It means that the Content-Resolver has a special job, what is it ?
ContentResolvers are designed to create an interface for applications to securely access and/or modify data in other applications which run in separate processes. It creates a common contract for content consumer registrations, requests, updates, binding, etc. to be passed between a particular ContentProvider and the content consumer that runs in another process.
Creating a proper and durable interface for transferring data across processes in Android is complex. If you had to create your own IPC client then it would be time consuming and you would likely overlook important aspects of the implementation. A "direct" connection to the ContentProvider is what a ContentResolver provides, but with a lot of the complexity already addressed. It is a convenience to already have that created and well-defined on both sides.
This is a little like a postal delivery system. If you use the postal service, you put an address on a piece of mail and the postal service (ContentResolver) finds the appropriate place to deliver the message (ContentProvider), and then returns any messages to you in response to your message. Your postal service will also handle new address registrations, tell you if the address is invalid, if the package is too big, if the recipient at the destination address is not accepting messages right now, waits if the message queue is full, handles situations where the vehicle carrying the message breaks down, etc. And in the case of ContentResolvers there are a few other things.
You could handle all of that yourself but when you think about real-life message delivery, it's not simple and much easier to use something built by someone else. Likewise for interprocess communication (IPC) in Android. It's not a perfect analogy but hopefully that helps clarify the need for a ContentResolver.