The first goal, as usual, is to relax. Focusing your attention on pleasant, stress-free imagery and sensations helps to take your mind off of whatever is going on in your everyday life.
I also think that visualization is helpful for people who find it difficult to clear their minds completely for meditation. It can help to stop your mind from wandering by giving it something to do, even if the “work” is pleasant imagery.
I use a lot of light and colour in my visualizations not only because each chakra is associated with a colour but because the different colours (which are just different frequencies of the electromagnetic field) have different effects, especially on how we feel. The feeling you get imagining yourself sitting in a meadow under a full moon is different than the feeling you get imagining a ray of red light passing through your body or seeing yourself sitting in a cave surrounded by different shades of green light.
Visualizations are an enjoyable way to take a mental time out. You don’t need any material or equipment, so you can do them anywhere and you can do them for as long as you like, whether it’s for two minutes while you’re waiting for your lunch order, 15 minutes while you’re on the train, or for a few minutes before you fall asleep.