Some people have problems with their back or other parts of body. I for example had a stomach rupture when i was a child, which was probably stitched by not very qualified doctor, and i can't do squats with more than 100kg. My legs could easily lift more, but my lower stomach feels like it's going to rip apart and i'll have to collect my guts from all over the gym. Maybe it's just pain and not an actual problem, but i don't quite want to find that out.
I don't understand, but I apologise either way. Edit: Have been informed by Fuse that it's because of my avatar. Can't believe I missed that. Too much masturbation.
You're absolutely right, they support your back, prevent injury or stop yourself from making an injury/problem worse like yourself. They are good for rehabilitation or people who are too weak due to injury to use the correct technique with free weights, as free-weight exercises with a poor technique can be dangerous. If your lower stomach feels like it is going to rip apart then be cautious, however they will often feel tight where they've been stitched and may actually be stronger than they were before. You could try doing bodyweight core exercises like planks and side planks to strengthen it and improve core stability before going on to try heavy squats again. Sorry for OT!