Why abstract classes are used??

Answer 1

Usually abstract classes are used to define interfaces and it is required to have at least one pure virtual operation (method). You want to define a abstract class when a instance can not be created and is used only for inherting from it. When you expect from it's child to have a behaviour as you expect. Check more about iterators and iterable collections.

Answer 2

An abstract class can be used as a type of template for other classes, and most commonly used for inheritance.

The best example:
abstract class Animal(){}

then you can extend child class such as: dog, cat, fish.