What's the point of mixing the same shader types? Many time I've seen some material using mixing of two same nodes like this: 

 

Is there any difference from using this?
They are not equivalent. Mixing two different roughnesses produces a subtly different distribution of angles at which light can be scattered:



Note the shaper highlights on the right hand example (mixed roughness).

To visualize what is going on, take this (very approximate) example:



The length of the arrows represents the amount of light that gets scattered in that direction.