Cats eat flies primarily because of their strong hunting instincts. Even well-fed domestic cats retain the predatory behaviors of their wild ancestors. Chasing and catching small, fast-moving objects like flies satisfies their natural urge to hunt. While flies aren’t a significant nutritional source, the act of hunting provides mental stimulation and physical exercise. In most cases, eating an occasional fly isn’t harmful, but it’s wise to ensure the fly hasn’t been exposed to pesticides or other toxins.