1)Interface totally keeps away that implementation 
knowledge from client.
2)It supports us to change their behaviour dynamically.It 
means that, it will act depends on dynamic specilization(or 
substitution).
3)This gives very good abstraction about the object to 
client.
4)It avoids client broken, even developer made some changes 
on implimentation or add new specilization(new 
implementation).
5)So it gives open way to extend and implementation.