Answer 1

A simple way to think why you would need a abstract class is to ask your self can I create an object of this in real life.
there are many things which you cant to name a few would be a
Machine,Animal,Mammal,Fruit,Vegetable ect ect...
so even though you dont know what a Animal looks like you can still get a idea of what a animal might do
So these should be abstract classes which will have the basics of what your object/classes will extend from.
For example
Abstract class Animal{
//variables would be
they all have names
they all have a colour
//methods/overridden methods would be.
they all can sleep()
they all can wake()
they all can eat()
they all make sounds()
}
And then you extend your diffrent types of Animals from the abstract class these will be the real life objects like cat, dog ect..
Hope that helps.