There are indeed many people in this world who are born with a great voice and can instinctively sing well. However, that doesn't mean that these people do not need vocal training, especially if singing is a hobby that they do frequently.
Just think of the vocal cord as your biceps and triceps. Just as you need to spend time strengthening your biceps and triceps in order to maintain their shapes; similarly, you need to strengthen your vocal cord in order to keep it healthy and prevent it from damage.
Strengthening the vocal cords isn't just about practising - in fact, practising in the wrong way can back-fire and easily lead to damages that are sometimes irrecoverable. What you need is someone to listen to you and correct your mistakes when you make it so that your body can make corrections immediately. That's why formal vocal training is so important. It helps you develop the right techniques to prevent damage to your voice.