Abstract classes and interfaces are complementary.
For instance when creating an API you will want to present interfaces to the client, so that you may always completely change the implementation whereas he does not have to change its code and the user does not rely on implementation when building using your API but just on methods contracts.
Then you will have abstract classes partly implementing these interfaces, in order to
- share some common code, which might be used in all (or almost all) implementations for interface, which is obvious
- provide default behaviour which could be overridden in 'real' implementations, for instance a toString() method using interfaces methods to create a textual representation of the implementation
- preserve implementations compatibility after interface changes, for instance when you add a new method in your interface, you also add a default implementation in the abstract class so that implementations (for instance those made by the user) extending the abstract class still work without changes
