Well, they're arguably necessary to show that there's no racial discrimination going on in organisations and so on, but only arguably, so I'm still not sure.
As for your point about nigger - the same is basically true of any swearword, so you get to the somewhat problematic question of whether a word itself, independent of any intention on the part of the speaker, can be offensive. Oddly, it seems that the answer is yes, as many people genuinely don't like to hear swear words, but nobody's entirely sure why this is. Steven Pinker has suggested that people necessarily link swear words to unpleasant thoughts/feelings, and so when they hear the word, they're forced to go back to that unpleasantness, which is a good a guess as any. Of course, if you think about it logically, there's nothing inherently offensive, but that doesn't reflect the way people behave.