What is the purpose of the AppCompat library in Android app development?

The AppCompat library in Android is used to ensure compatibility of new features and user interface styles with older versions of Android.

It allows the use of modern Material Design components and new APIs, even if the app runs on devices with older OS versions. This is achieved through backward compatibility implementation and providing unified classes and widgets.

For example, with AppCompat, you can use Toolbar instead of the standard ActionBar, and also apply Material Design themes and styles on devices with Android below 5.0.