Higher fire rate and larger magazine means you can make more people dead, faster than with a regular Heavy Bolter. With fewer trips to the resupply box. That's worth something. It also looks cooler than the regular Heavy Bolters, and looks are super important when you're murdering fools in Khorne's name. The Teeth of Khorne won't make you some kind of unholy murder machine by itself, compared to the normal Heavy Bolters, but damn if it doesn't help.