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Because of the backward pawn on c7, White has the advantage. However, if Black can play ...Nd8-e6, his position will already not be so bad – in the first place, there is the idea of ...c7-c5. To stop this freeing break, White can play b2-b4, but this move restricts his own bishop, and he does not wish to do this unless absolutely necessary. Therefore, while he has the chance, White carries out a favourable exchange of dark-squared bishops:

17.Bg5! Bxg5 18.Nxg5 c6 19.Rac1 Rc8 20.Qd3!

White forces ...g7-g6, because simple variations show the benefits of weakening the square f6.

White has won a pawn without any compensation, and the rest is technique.

White’s task is to prevent the freeing move ...c6-c5. The white knight aims to control the c5-square, at the same time freeing up f3 for his bishop, from where it can successfully attack the enemy weakness:

A mistake, after which White also obtains the b4-square, which makes it even easier to control a whole complex of dark squares.
He should have chosen 19...Be7, and then ...Rd8-c8.

One weakness on c6 may not be enough to win, and so White intends to open a second front – by playing b2-b3, he gets the open a-file to enter with his rooks. If this plan had not been available, White would have advanced his kingside pawns to seize space, and combine threats on that flank with pressure on c6.

White has more space, and therefore he refrains from exchanges that do not bring him any concrete benefits.