Keith's explanation hit the nail on the head
AFAIC. Cardio will help lose weight and is great for cardiovascular (heart and blood delivery) conditioning, but it won't strengthen muscles. You need some sort of resistance exercise for that. When you strengthen the muscles, the increased muscle mass will help your body burn more calories even when it's at rest, which will help you to actually maintain your weight loss. Without that, you run the risk of gaining weight back if you cut back on the cardio.