Multiple things. EGR, the scr, and particulate filters are not super reliable, and when they fail, either your truck goes into limp mode or if your EGR cooler f

Multiple things. EGR, the scr, and particulate filters are not super reliable, and when they fail, either your truck goes into limp mode or if your EGR cooler failed and your engine sucked in coolant, you're buying a new engine. They also give you slightly better fuel economy and more torque/horsepower. Then there's the real jackasses (like my cousins) that delete all that stuff along with the muffler and deliberately blow clouds of black smoke because deep down they are deeply insecure people and suffer from undiagnosed depression and anxiety.