The mind automatically works in images, every memory you have conjures up an image of the memory, the other senses like taste, smell, sound and touch are also a part of the image, but your first sense of a memory is sight. When you want to make permanent changes to your life, improve your thought processes towards yourself or simply work towards a more positive existence, using meditation visualization you will find that meditation combined with visualization works faster than simply using one or the other alone.
With a practice of meditation and visualization you will be concentrating on what you want to achieve and visualizing it, which will make the body and mind believe it to be true and you will start to live if as though it is true. A simple example is for people who want to lose weight, if you meditate and visualize that you are thinner, healthy and full of energy you will start to eat healthy, feel more energetic and do things that will help you to get to that thinner frame.
One of the biggest mistakes people make when it comes to meditation visualization is to visualize something that they do not believe to be possible, for example if you want to lose weight, and you visualize yourself as a size 10 when you are currently a size 30, you do not believe that you will ever achieve the size 10, so your visualization is unrealistic, you will find that you doubt your ability and land up failing. Break your ultimate goal up into smaller goals, so using the same example, visualize yourself as a size 26, that is a lot easier to achieve than a size 10, and soon you will be that size 26, then you can visualize yourself as a size 22 and so you go through the process to your ultimate goal, but each time you achieve the smaller goal your confidence will improve and you will feel better.
The visualization part of meditation is simply another tool to help you achieve the goals you set out to achieve, the mind and body will follow the input you give them during your meditation visualization.