Specificity. "The" implies one specific subject. "A" implies that there are multiple instances of the subject, and in many cases such subjects are used to categorize things. "The dog is a golden retriever." --. "The dog" is the specific dog being mentioned, and "a golden retriever" is its category.
"He was the victim" implies he was the only victim. "He was a victim" implies that there were other victims.
What's your first language? All of the european languages that I've studied have this feature.