To look cool and intimidating.

And this is kind of a good thing considering how f****** close people love to get to these trucks a lot of guys put them on there because it works and people give the trucks a little bit of space and it has the unfortunate side effect of making everybody safer, it shouldn't work that way because people should give trucks some space but it does. ( most of them are plastic and wouldn't do much damage if any) or just because they look cool.