"I don't see why White can't just handle the top"
"will play 4 at "a"
stopps the support from the outside group.
This brings time to the B corner with , to help himself.
Instead of this 10, white will play pretty devilish tesuji at 11 himself. Black cant play at 10 (trying to form an second eye, cuz it will be selfatari (white will capture big clunch of black stones at point above 11), and if black capture it at "a", then white play 10 and black is reduced to one eye.