Why different types of object reference is allowed

Question

I wonder why it is allowed to have different type of object reference? For example;

Animal cow = new Cow();

Can you please give an example where it is useful to use different type of object reference?

Edit:Cow extends Animal

Answer 1

This is an inheritance 101 question.

It allows objects that share common functionality to be treated alike.
It also allows specific implementations to be supplied at runtime that are subclasses of an abstract type.

I could probably ramble on for ages. Perhaps thus question is just too broad to answer here.