Personally, I feel that wearing a belt provides more of a psychological advantage than a physical one. Let's face it, having a weight belt tightly cinched around your waist makes your physique look better by giving you the appearance of an enhanced V-taper, and makes you feel more "serious" about what you are doing. A belt also provides a feeling of "tightness" around your waist, which would seem to protect your lower back while weight training, but does it? Here are some reasons that wearing a weight belt may not always be the best thing...
1. Weight belts can weaken the muscles of the trunk by causing too much intra-abdominal pressure. In an effort to try to prevent this, the body may force the abdominal and lumbar muscles to relax while lifting, which over time can weaken these muscles and cause back problems.
2. Weight belts can accelerate degenerative disk disease by restricting the natural motion of the lumbar spine.
3. Weight belts can cause poor posture by compressing the lower abdominal region. This will force you to restrict your breathing to the upper chest area which can throw off posture, as well as cause tension headaches and upper disk problems.
4. Weight belts alter the natural biomechanics of the spine, especially during rotational movements. This can weaken the smaller, stabilizing muscles of the spine, which can result in back pain and possibly osteoarthritis. With this information in mind, my advice to you would be to avoid the use of a weight belt, except during your heaviest sets of compound movements like squats, bent rows, deadlifts, military presses, etc. At other times, allow your body to stabilize itself.