Someone discriminating has fine-grained judgment and can tell, for example, the difference between pretty good cookies and incredibly great cookies — a valuable skill.
There are many areas of life where being discriminating can help you. A discriminating TV watcher will only watch the best shows, while someone undiscriminating will watch anything that's on. When eating, being discriminating is especially useful: why eat food that's badly prepared or unhealthy if you don't have to? A discriminating person can pick up on the small differences between things and use those differences to make better choices. We should all try to be more discriminating.