Swearing is a way to relieve tension. It does this in a few ways, including the actual physical production of the words in your mouth. Consider that there are lots of hard consonants (eg, t, k, f sounds, as opposed to something like h or sh) in English swear words. These sounds require you to put parts of your mouth together, or flex the back of your throat, you might draw out those consonants quite a bit, etc. That can relieve physical tension in a way that "sh" or "l" can't.

The other thing that swearing does is relieve psychological tension by allowing the speaker to cross the taboo. The fact that a swear word is taboo in and of itself does this. But what actually makes it taboo has to do with what society is worried about. A hundred years ago, English speakers were far more likely to be deeply religious. Thus, it was frowned upon to do things like "say the Lord's name in vain" or talk about other religious things in a non-serious, profane (meaning not religious) context. This is why profanity has actually come to mean a curse-word.

In the modern day, English speakers are not as religious, on a whole, as we used to be. Instead, sexuality (and to a lesser extent human filth), became much more taboo (something that developed over the course of the 19th century). So, slowly, parents started spending less time worried about whether a child yelled "Oh my God!" and a lot more time worrying over whether he yelled "Oh, shit!" Thus, a shift in which swear-words feel more taboo.