Why is Amir's time with Farid significant? Why would Hosseini include him as a character in the story?
This is an interesting question because Farid is introduced as a means of transportation, so it is not immediately obvious what part he plays in the story. However, he is important for a few reasons.
First, his character in contrast with Amir's character shows us how Amir has changed in his twenty years in America. Amir tells Farid, "I feel like a tourist in my own country" (231). And we can see how Farid knows his way around, what to do and where to go, in a way Amir cannot manage.
Second, Farid's attitude gives us a glimpse into the attitude of Afghans. Farid clearly disapproves of those Afghans who have left for greener pastures. And you can also see his disdain for Americans in his conversation. Also important is the understanding we have about socio-economic divisions in Afghanistan. Farid has been through hell. He is a poor man who is missing two toes and three fingers. He has wives and many children to support. When Amir says some part of him will always be Afghan, Farid snickers and points out to Amir what a privileged life he had led when he was in Afghanistan, not the life of the typical Afghan.
When Farid learns of Amir's reason for being in Afghanistan, his attitude changes,and he becomes important in other ways. He is a kind of "tour guide" to Amir, so Amir is able to see what has happened to his country. (And in fact, in this sense, Amir is a stand-in for the author, who is quite active in several important Afgan causes.) Farid also becomes an active participant in the search for Sohrab and ultimately rescues Amir and Sohrab, after Assef nearly kills Amir.
So, all in all, Farid, the lowly taxi driver, is a most important character.