I had searched about it but still not clear Why it is necessary. I know the description but why do we need Microsoft Expression Blend (I am asking in terms of WPF designing)? What does it have that we don't have in Visual Studio without it.
(I am not asking personal opinions, I am asking what features does it have to contribute usage of Visual Studio or just to Developer?)
Blend has a timeline-style storyboard editor, in VS you have to figure out all the keyframes by hand. It is also easier to do vector graphics and 3D layout in Blend.
Basically, any kind of advanced UI design tends to be easier in Blend, and it is easier for artists and other non-developers to use. I would personally never give an artist Visual Studio to design a UI for me (mostly because everything would be broken :) ).