Why is Pip and Biddy's conversation in chapter 17 significant to the development to the plot of the story?

This chapter lays out what has remained unspoken for some time to a somewhat relieving affect: Pip comes right out and says he loves Estella and that, foolish even to himself, he wants to become a gentleman to win her over. The discussion, symbolically, takes place among the marshes, which have, throughout the novel, represented Pip's past as well as his social position as a blacksmith's apprentice. The pastoral peacefulness that accompanies Pip's walk with Biddy is contrasted with the ships in the river, that Pip has always associated with some far away, expected future. Pip himself states his frustrated state when he says he wishes he were happy in his current position, including having Biddy close, but he is forever looking toward some impossible future.