Remember that this for evaluating future states of the board. A state of the board where the opposing king is taken (or technically just checkmated), yielding a

Remember that this for evaluating future states of the board. A state of the board where the opposing king is taken (or technically just checkmated), yielding a win is highly desirable. In this case K' is zero even though this can't happen in a middle of the game. By having such an evaluation function the computer is encouraged to make moves that reach this state.