As the name says, this type of dog is produced without the use of grain.  But why should you feed your dog a grain free diet?

Grains are cheap fillers.  The reason many dog foods contain grain is because grains are incredibly cheap to source.  The dog food manufacturers of these foods are increasing the volume of the food, with low value ingredients (in cost and in nutritional value).  The end result for the pets who eat these foods, are diluted, lower quality meals.

Grain free foods are typically more expensive than non-grain free foods, because high quality ingredients are used.

Dogs aren't meant to eat grains.  Nor do dogs want to eat grains.  You could leave a bag of grain in a room with a hungry dog for an hour and the bag of grain would go untouched.  A large plate of cooked meats?  The dog would waste no time in eating this.

A dogs digestive system does not handle grain very well.  For thousands of years dogs have been eating meat, and their digestive systems are built for digesting meats.  Grains are commonly not highly digestible for anyone, but this is even more so in dogs.

Grain free foods generally include high meat content for these reasons.