I've never heard of a posture belt, unless you mean a weightlifting belt.
Assuming you don't mean weightlifting belt, if it's something that physiotherapists prescribe, then it may be good for posture, at least in some instances.
If you mean weightlifting belt, then it won't have any significant impact on posture outside of the gym. It may be helpful for keeping good posture while lifting, because having something touching your body may give a strong enough proprioceptive cue to help you focus on your form, but if you haven't felt good posture without the belt, then you won't know when you've got good posture with the belt. Plus, weightlifting belts work by contracting your abs outwards against the belt, increasing intra-abdominal pressure. Drawing your stomach inwards will make the belt a useless accessory (also, drawing your stomach in isn't necessarily good posture - most of the time there should be enough tension in the transverse abdominis to keep the gut from hanging out, but you shouldn't be doing a stomach vaccuum 24/7, either.
At the end of the day, unless you need to be physically forced into a good position, like the others have said, it's gotta come from you.