we use interface for user to create their own method out of it. User can implement that interface on any of their class.
E.g. WebDriver is an interface which is implemented by classes, RemoteWebDriver, FirefoxDriver, ChromeDriver and many more. So you can easily declare a variable of type WebDriver and initiate it with any of above classes.
You use interface when you want to enforce rule/create blue print of your architecture. Also you are allowing user of the interface to implement another interface as well.
e.g. RemoteWebDriver implements all below interfaces.
When you make abstract class, you make sure that user extends that class, user should not create new instence of that class. Again that will act as a blue print for you.
E.g. BaseActions, BasePageObjcts. these classes will have implementation of common reusable methods and variables
Hope that helps.