Yes, a back support belt can improve your posture, especially if it's worn at night. During the day, back support belts help you maintain correct posture with a straighter back instead of slouching over. For maximum benefits of daily posture, it's recommended that you strengthen and retrain your muscles, otherwise the belt will be less effective in correcting posture.
You should be careful not to wear your posture support brace longer than your doctor recommends because your body may get used to it and start to rely on it. Some back braces may cause more harm than good so make sure to speak to your doctor to make sure you choose the right one for your injury.