Political corectness is a social tool that evolved to promote respect among the members of a society. It pressures people to avoid offending others, which is a form of respect and it fills a gap left by the decreasing influence of traditionalism.
We no longer show respect for blood lines (the royal family n lords are social relics), we show less respect for being part of the same extended family(clan) and we show less respect for those in higher classes. In other words, respect in our society is not granted automatically, it must be earned (that by the way is part of the explanation for our celebrity culture).
In conclusion, respect is crucial part of a society, our old rules for granting it are obsolete so we made new ones. Political correctness is one of them and no matter how much some of you whine about it, it's here to stay because we need it.
Why so many people seem to hate a concept that basically says 'try not to be an ahole' is beyond me but if I were to guess I would say it threatens their identity.