Two reasons:

1. I live in Utah, where the Mormons are, and they get very offended by swearing. Although there are some ways in which I will definitely not accommodate their religious beliefs, I also think it's healthy to meet other people at their comfort level (if it's reasonable to do so). On the one hand, I understand the idea that we shouldn't have to change who we are in order to make other people comfortable. On the other hand, I do think that if you take that idea too far, it can be a kind of antisocial behavior. When in Rome, as they say.
2. It has more impact if you don't do it often. Think about a Quentin Tarantino movie. By the time Samuel L. Jackson has said "fuck" for the 157th time, you're just used to it. The word doesn't even stand out anymore. But now consider the end of The Princess Bride, which has one swear word in the entire movie: "I want my father back you son of a bitch." WHA-BAM! Hits like a freight train every time!

For the follow-up questions, kind of the same answer to both of them. I feel like not swearing -- or, swearing less -- requires me to be more precise when I'm criticizing something. Instead of just saying that something was "like shit", I have to give a more specific criticism. So that's the change that it has made, and no, it hasn't stopped me from expressing something.