You should only ask people to authenticate in exchange for value, such as the ability to personalize the experience, access additional features, purchase content, or synchronize data. If your app requires authentication, keep the sign-in process simple and unobtrusive so it doesn’t detract from the enjoyment of using your app.
Delay sign-in as long as possible. People often abandon apps when they are forced to sign in before doing anything useful. Give them a chance to fall in love with your app before making a commitment to it. In a media streaming app, for example, let people explore your content and see what you have to offer before they must sign in to play something.
Explain the benefits of authentication and how to sign up for your service. If your app requires authentication, display a brief, friendly explanation on the login screen describing the reasons for the requirement and its benefits. Also, remember that not everyone using your app has an account from the start. Make sure you explain how to get one, or provide a simple in-app way to sign up.