EndgameTesuji2/Why is the marked stone necessary

Block  

Bill: With the block at Black can secure the corner.

Jump  

Bill: If Black omits , White can play the clamp at a. After Black makes the same amount of territory as with the block, but White has ko threats, starting with the cut at b.

Block inferior  

Bill: When Black has , the block is inferior to the jump.

Jump superior  

Bill: When Black has , is not sente. In fact, it is reverse sente.