Life In 19x19

I was replaying this game and am confused about the ko that develops around L18 & L17. It begins in earnest on W140 and continues up until B195.



My question is - what is the point of this particular ko?

My infantile understanding of ko situations is that one or both players has a group in danger, and because losing the ko incurs a cost, the opponent can play threats at various appropriate places on the board, and that this goes back and forth until one player runs out of more valuable or equivalent value threats.

But in this particular case I do not see the danger. Neither the black group at top center or the white group at top right ever seem to be in any danger of being killed. So why is this ko valuable?

This is an example of sente ko, in which one player (Black in this case) "loses" the ko by taking gote while leaving the unresolved ko mouth on the board. There is a possible smaller ko fight there later. See http://senseis.xmp.net/?SenteKo

White's sente threat is to make a ko for the life of the Black group. It is also a kind of sente ko with a double ko shape. Black "loses" this ko by taking the other ko of the double ko. That results in another ko for the life of the Black group. Black has sacrificed a few stones and a few points in the corner to keep fighting for his life.

Often sente kos are not fought at all, or are contested only once, to force the winner to use up a ko threat before the loser takes gote. But sometimes, as here, there is a protracted ko fight.

Just in case it wasn't clear, if white connects at m19 to save the m18 stone then black will tenuki as his group is already alive (white is short of liberties to play n19). So the ko is not fighting about the fairly small endgame of the l17 or m18 stones, but whether or not black needs to defend in gote at n19 to avoid a ko for the life of his group.