There are a few occasions were you would need one.
1. Rope needs to be cut
2. Something needs to be cut
3. Something to mess with when bored
4. Protection
5. ???
6. Knowing that you can be safe if a dude tries to rob you.
When do you ever need to cut rope? Also, most times when I need to cut something, I have scissors lying around or I don't need any tools. When I'm bored, I tend to mess around with my phone, and that's not a real purpose for a knife. Four and six are the same thing, and seem a little paranoid, if you ask me.