Why using delegate and example ???

Answer 1

read this article:
https://msdn.microsoft.com/en-us/library/900fyy8e.aspx

Answer 2

Delegates is a useful, flexible and powerful technique. I will give you a basic explaination but reading the c# reference page would give a better explaination.
Firstly a delegate is basically reference type variable used to reference methods, you can think of them as C++ function pointers but C# delegates are type safe. Delegates can also be used to pass a method reference to another methods argument. Like i said they are flexible and you can do alot with them. Read the microsoft c# reference page for more info.