Even though they have similar names and involve connecting to a remote computer, Remote Desktop and Remote Assistance are used for different things.
Use Remote Desktop to access one computer from another remotely. For example, you can use Remote Desktop to connect to your work computer from home. You will have access to all of your programs, files, and network resources, as if you were sitting in front of your computer at work. While you are connected, the remote computer screen will appear to be blank to anyone at the remote location who sees it.
Use Remote Assistance to give or receive assistance remotely. For example, a friend or a technical support person can access your computer to help you with a computer problem or show you how to do something. You can help someone else the same way. In either case, both you and the other person see the same computer screen. If you decide to share control of your computer with your helper, you will both be able to control the mouse pointer.