Atticus tells Scout this so that she will realize that his choice to defend Tom Robinson is a moral choice.
Atticus says that there is a case in every lawyer’s career that “affects him personally” (ch 9, p. 100). His point is that he takes the case seriously because it requires him to make a moral decision. He has to stand up not just for Tom Robinson, but against the town. He has to make the choice to do what he thinks is right.
“Simply because we were licked a hundred years before we started is no reason for us not to try to win,” Atticus said. (ch 9, p. 101)
Atticus is telling Scout this so that she will learn a life lesson. It is a person’s responsibility to stand up for what he or she believes in. You need to stand by your choice, if you think it is right, even if it is not the most popular choice. This is the strongest lesson that Atticus teaches Scout. He leads by example, showing her that it is important for a person to do the right thing. This is the only way that racism can be overcome.
It is also significant because "Atticus represents the moral backbone of the town of Maycomb" (enotes, character analysis). By standing up for what he feels is right, he is showing the town what is right. Ultimately this does make a difference, because the jury does at least consider Tom Robinson's potential innocence.
To read more character analysis, see here: http://www.enotes.com/to-kill-a-mockingbird/character-analysis
To read a summary of this chapter, see here: http://www.enotes.com/to-kill-a-mockingbird/chapter-9-summary-analysis
Enotes. "To Kill a Mockingbird
: Character Analysis." Enotes.com
. Enotes.com. Web. 17 May 2012. <http://www.enotes.com/to-kill-a-mockingbird/character-analysis>.
Lee, Harper. To Kill a Mockingbird. New York: Warner, 1986. Print.