after 10, black clearly needs to capture.
And now white plays somewhere down the side to use the wall.
sente, so it is not so easy to be sure it is really worse.
black prefer to have it settled now?
Herman is right. The descent is White's play, not the hane.
White can play the descent here, too, aiming at Wa.
However, threatens to play at , so that, unless White plays a descent immediately, White must play the atari, leaving the standard diagram.
Now White's connection is not as good as either descent.
That's why the diagonal play is standard.
The exchange in the last diagram labeled Tewari is not good for white. It is an endgame exchange after which white must defend. If he doesn't defend, he shouldn't play it. If white wants to play here now to strengthen his group, he should play the descent.
So by playing the joseki move, black forces white to make an unfavorable exchange. White can't ignore because then the black hane is very strong.
Thanks for the responses, guys. Interesting stuff.