I've never understood this either. I cannot understand why swearing or partial nudity are considered so harmful to children, but most people think nothing about them being exposed to violence.
I know many people feel this way, but I guess I just don't quite understand what harm is being done by occasionally cursing in front of a child.
I think I equate it to some people who believe alcohol should never be consumed in front of a child, which I also don't understand. Children are capable of understanding that alcohol is something reserved for adults and I believe it's healthy for them to be exposed to adults enjoying a drink in moderation on occasion. When it is always hidden, I think that's what makes it more "forbidden" and they may not learn the tools to use in moderation or appropriate situations when they become adults themselves.
I would think the same would apply to swearing. If children have never been exposed to it, how do they learn to navigate social cues and determine what situations are appropriate for swearing and which are unacceptable?
Of course the "f-word" means more than "fudge". That's how swearing works. There is an element of taboo which gives those words greater weight. Even within swearing, different words carry different weight. This is understood in each society and is how you can instantly recognize the severity of emotion being expressed between two different swear words.
There are lots of reasons why people swear and it's not just to "look cool". Here are a few examples, most backed by scientific studies:
-- swearing is cathartic
-- to express a direct, succinct emotion (both positive and negative) in a way that any other words would not get across
-- pain relief / the ability to endure greater pain (I have a great lamaze technique involving the f-word

)
-- to put others at ease and gain access to a group (this is often helpful in workplace dynamics)
-- humor
-- because it's a natural and universal part of human existence
ETA: There are tons of articles and studies on the subject of swearing, but here's a brief one you may find interesting:
Why We Swear: Profanity Is Powerful | Time