I don't know that everyone writes a memoir for an audience. Sometimes it just needs to be written for the author. McCourt was not well known before this was published, and I don't think he ever dreamed it would become so critically acclaimed. So I think his primary motivation was for himself, to put his troubled history into some sort of constructive context. The fact that masses of Americans were able to relate to it was simply a pleasant surprise to him.
There are many reasons why a person writes a memoir. Unless an author comes right out and tells in an article, interview, or journal as to specific reasons why he wrote the novel, all we have is speculation. For instance, Elie Weisel wrote NIGHT as a type of therapy...to "heal" from the injustice dealt him of losing his entire family to the Holocaust and to move past the nightmares that haunted him of murders and mistreatment by the SS and the Nazis in WWII. Undoubtedly, he also did it to teach others about the experience he had, to add to the growing collection of Holocaust testimonies. This is just one more way to prove to all the fools out there who swear the Holocaust never happened that they are dead wrong.
I am certain that McCourt had all these reasons in mind as he wrote his memoirs as well. He wanted to teach about the strength of the human condition--people endure much suffering and still, for the most part, come out OK.
As a die-hard McCourt fan that I am, I was surprised to see the controversy regarding Angela's Ashes, 'Tis, and Teacher Man, especially since such controversy came out of Limerick, and the claim that his memory and recall of events was "too vivid" or else "too exaggerated" for it to be accurate. He, however, won the Pulitzer for an Autobiography, and he indeed teaches, if anything, about the struggles of a poor Irish family haunted by the evils of alcohol, by an unfair society, and by the drastic changes that they experience once they make the cross to America.
In my opinion, if his memoir had gone through the same scrutiny as, for example, A Million Little Pieces they may have withheld the enthusiasm for awarding a Pulitzer right away. When you read the series (all three of the McCourt's Angela's stories) you feel compelled, inspired, sad, happy, hopeful, depressed. He does accomplish THAT, and teaches about struggle and strength of character in between. Whether his stories were as accurate as he claims them to be, that is something for him to write about again later. :)
His primary reason was probably to discover the value and meaning in things he had done over the course of his life. You can view his interview with Brian Lamb on why he wrote the books that he did at :
C-Span also has other wonderful programs on authors.