Determining the existing state of UI Automation support for your custom UI class
Before you attempt to implement an automation peer for a custom control, you should test whether the base class and its automation peer already provides the accessibility or automation support that you need. In many cases, the combination of the FrameworkElementAutomationPeer implementations, specific peers, and the patterns they implement can provide a basic but satisfactory accessibility experience. Whether this is true depends on how many changes you made to the object model exposure to your control versus its base class. Also, this depends on whether your additions to base class functionality correlate to new UI elements in the template contract or to the visual appearance of the control. In some cases your changes might introduce new aspects of user experience that require additional accessibility support.
Even if using the existing base peer class provides the basic accessibility support, it is still a best practice to define a peer so that you can report precise ClassName information to UI Automation for automated testing scenarios. This consideration is especially important if you are writing a control that is intended for third-party consumption.
Automation peer classes
The UWP builds on existing UI Automation techniques and conventions used by previous managed-code UI frameworks such as Windows Forms, Windows Presentation Foundation (WPF) and Microsoft Silverlight. Many of the control classes and their function and purpose also have their origin in a previous UI framework.