How effective is "political correctness"?

disclaimer: I do not intend to offend anyone here or elsewhere

Does changing something to make it “politically correct” have an effect on the offensiveness of the topic? Why is “African American” politically correct even if the person in question is not from Africa? Also, is a white man living in America who originally was from South Africa considered “African American”? Why are most white people in America not called “Anglo Americans” or “European Americans”?