Why do people say “no offense” right before they’re about to offend you?

I don't really like to talk about myself (had to put that out there...) I'm extremely random and my sense of humor is a bit hard to understand at times (can you tell by my headline?) If I wanted to describe myself I would say I am an easygoing person that chooses to enjoy the little things in life. I enjoy doing pretty much anything as long as it's with the right people. I plan on attending medical school in the future so studies tend to be my focus these days. A sense of humor is a must because I find it important to be able to laugh about things (including at myself). Family is very important to me. Oh and I'm extremely sarcastic and have been told by many that I have a pretty strong personality (most mean it in a good way...I think). I don't love easy, but when I do, its with all my heart. Hmm the kind of person I'd like to meet....just an overall good guy (already mentioned the sense of humor and sarcasm). Believe it or not those are hard to come by these days.