A knife sharpener is any device that restores and maintains the cutting edge of a blade. In a nutshell, the blade is rubbed or dragged along a hard, rough surface, removing minute amounts of metal until the cutting edge is thinned and sharp.
Knives dull with repeated use, and a dull knife is hard to work with.
Not having a sharp blade can slow you down, and make it difficult to perform essential tasks associated with barbecuing, such as deboning, filleting, slicing, and carving.
A dull knife can also slip during cutting, creating a higher potential for injury. So a sharper knife is actually safer.