Why do some people use offensive words, even after they know they are offensive?

While reading through some questions here, I noticed that people sometimes continue to use a word that others have specifically said they find offensive. I’ve seen examples of this in day to day life, too.

Words like retarded, fag, and slut are very distressing to some people. I understand that not everyone feels that same level of distress, but, once you know that other people are offended, why continue to use them? Is the goal to be hurtful? Is there some deeper motive?