I tried a belt on a whim, not because I was unable to lift the weight or even thought it was necessary. I wanted to see what it felt like. When I tried it, I understood. All it does is let you get firmer in your abdomen. And it sort of teaches you what that firmness should feel like if you are someone who hasn't been doing that very well. So it's not like you need it at a specific weight. Just try it out on your final set and you'll understand what it does. Then you can decide if you want to use it.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 192.5lbs, press 75lbs and deadlift 210lbs