1. Because it's a good way to release pressure without hurting anyone. For example, today I panicked about something, and I think swearing to myself helped me calm down.
2. I almost never use swearwords as intensifiers, but when I do it's fun.
3. When a Precision F-Strike is used, it can add a lot of emphasis to a sentence. (Warning: very sweary TV Tropes link)
After you die, how do you want your body disposed of? (burial, cremation, etc.)