Weigthlifting belts have been shown to increase intraabdominal pressure during lifting. An increase in intraabdominal pressure can reduce the amount of force your lower back muscles (erector spinae) need to produce to perform certain lifts. Such lifts may include the squat, deadlift, or over-head press. (Not doing these? You should.)
So why would a weightlifting belt be bad? If all the lifting is done with a belt, the abdominal muscles that produce intraabdominal pressure might not become properly trained or conditioned. If you are a weight lifter who has come accustomed to wearing a belt you are at a higher risk of injury if you attempt to perform the same lift without your weightlifting belt. Without those muscles conditioned optimally you are putting excessive pressure on the disks of the spine (now asked to handle more of the workload).
If your Doctor of Physical Therapist has instructed you to wear a weightlifting belt please do so, but a supplemental training program should accompany, which would include training muscle of the core on their own.
If you are a new or young weight trainer it is very important to learn how to properly engage your core on compound lifts such as the squat and deadlift before ever considering wearing a belt. As the weights get heavier down the road there maybe be a time to use one but until then the longer you can go without relying on a belt, the better.
How to practice:
The next time you perform squats take a look at what your stomach is doing. When you lower your body is your stomach getting pushed out? To fix it draw your abdominals in as you lower the body in the squat position. As you rise practice exhaling forcefully. Inhale at the top of the movement and draw the abdominals back in as you drop. Notice how much stronger you feel and how relieved your lower back will become.