We all know that Partial class allows us to split the definition of a class over two or more source files. Each source file contains a section of the type or method definition, and all parts are combined when the application is compiled. When it comes "Why Microsoft has introduced it?" we usually say "When working on large projects, spreading a class over separate files enables multiple programmers to work on it at the same time".
But when I had been asked this question, interviewer was expecting some different explanation i.e. Microsoft has introduced Partial Classes because it was their own requirement to produce the automatically generated source code.
Visual Studio uses this approach when it creates Windows Forms, Web service wrapper code, and so on. We can create code that uses these classes without having to modify the file created by Visual Studio.
So we can put some extra impression by adding above line in to our answer.