At the end of Lech Lecha prior to his bris milah, Hashem changes Avraham's name from Avram to Avraham. As per Rashi this was a promotion. He was raised from a chieftain status for Aram to the father of the world. With this promotion came responsibility.
Rashi on verse 18:17 (as do others) mentions that this land was promised Avraham and it is not right that Hashem should destroy 5 cities and decimate the natural enviroment of Avraham's inheritance with an explanation.
The other reason given is that Avraham is; "the father of a multitude of nations; should I destroy the children without informing the father who loves me? (Genesis Rabbah 49:2)"
So Avraham's fatherhood really extends to this evil nation. So G-d informs him. Shouldn't that be the end of the communication, after all Avraham is G-d faithful servant and accepts G-d's decisions.
It extends beyond that. According to Da'at Zekanim this is a warning to Avraham and his descendants. They must know that G-d is directly involved in the running of the world and if they follow the path of the Sodomites they too will be vanquished from the land. But Avraham's descendants must know even more. Unlike Noah, his selection involves a responsibility to his descendants, to provide them with exemplary actions.
Per the Tur : "Having been on record as praying on behalf of people who had already been condemned, his way of life of combining charity with righteousness, justice, would prove to be an acceptable role model for his descendants, and in fact, for much of mankind as a whole. It was also important to convey to future generations that even an Avraham, with his almost boundless agreed with G’d that there are people who are beyond the pale of redemption.
Is Avrohom's prayer pro forma? Is it an exercise in proving to the following generations his concern or virtue or did it have the potential to be effective? Ramban writes that: "either he will find merit for the sinners and pray that I spare them, or he will find none and then he too will desire that justice be done." Avraham can sway HaShem's decision without Sodom doing Tshuva! Who is Avraham that he has such strength! There is a hint in Yehayahu: