1) What is the point of creating a class that can'y be instantiated?
Just because something is not instantiated directly does not mean it is useful. An abstract class plays an important role in inheritance and can be very useful in class design of its inherited classes.
For instance, I have used abstract classes before to define the basic structure of what a class must conform to. Then I have defined inherited classes based on that abstract class and if I have missed a required method or something, it can be picked up by the compiler.
What this also allows you to do is associate inherited classes together which means you can then assume certain methods defined in the abstract class will exist in the inherited classes - which can be useful.
2) Why would anybody want such a class
Typically I use it for ensuring that a series of inherited classes have certain methods etc. For me it is really useful for design of a structure of a set of inherited classes.
3) What is the situation in which abstract classes become necessary?
I don't think an abstract class is ever necessary, but when used in certain circumstances it can be useful and help to simplify the problem you are solving.
