I seem to be seeing less of them than I used to, but a lot of people still use weight belts when they are weight training. The theory behind it is that by compressing your midsection, it will help increase intra-abdominal pressure and keep your lower back in a more neutral position. This will help prevent excess rounding or arching of the back and take some stress of the vertebral discs. While this may often be true, the weight belt is taking the place of your body’s own anatomical weight belt, the transverse abdominus. The transverse abdominus is your body’s deep ab muscle that helps to pull your abs inward and acts as a corset of sorts. When you use the weight belt instead, you don’t have to utilize your TA and it loses strength and endurance.
In many ways, a person can become psychologically or physically dependent on the weight belt when they are working out. There are times when the weight belt is necessary, such as powerlifting large amounts of weight on squats, deadlifts, strongman competitioins, and possibly bench press (professional powerlifters not regular bench press). Using it for bicep curls and lateral raises? Not so much. In my opinion, if you are not a professional powerlifter/strongman, if you can’t lift the weight without the belt, don’t lift it at all! Your body needs to adapt to the forces and stresses on your back and core muscles so it can adapt and become stronger. I am glad I am seeing less weight belts in use at the gym these days.