Why each case is evaluated if we ommit break even if variable is not matched?

Answer 1

Why... I can't answer that.

It is called fall-through: If there's no break, execution drops to the next case.

You can use it to bind several cases together as one.

case 1: case 3: case 5:
// Now the code for these three.

Answer 2

to have two cases with the same statements without repeat it

case 1:
//do1
case 2:
//do1

case 1:
case 2:
//do1