Because growling can be a precursor to aggression that results in a bite, growling is usually seen as a problem. But sometimes, the function of growling is to serve as a warning and that is not always a bad thing. When a dog is startled out of a deep sleep, she may growl as she jerks and wakes up. This warning growl is to tell you (and whatever possible predator may be in the area) to back off.
Dogs will also growl if they are bothered or pushed when they are in pain or afraid. This is a time to ask yourself if you should back up and let the dog calm down before approaching it again.
Warning growls (such as when someone unexpectedly wakes the dog) are in one sense good because they show that the dog has impulse control and it just does not just lash out and bite.
When growling is a threat that a bite is coming (e.g., when the dog doesn’t want to do something) it is time for training and the use of behavioral procedures such as reinforcement and desensitization.