Trucks tend to be high off the ground relative to cars. They also have bigger tires, and those tires are really good at sending things flying. If you’ve ever gone offroading or had your window crack, unfortunately, you’ve seen what those tires can do.
So adding mud guards helps protect your truck, the underside of your truck, and other drivers if you’re on the road. One of the reasons I’m pretty cautious behind semi’s isn’t just that they’re huge machines that weigh a ton, it’s that I’ve had my windshield cracked being behind one without mud flaps! Through experience, I’ve learned to avoid them. End of sob story.