The biggest impact I see is when you're trying to lift near your max on compound exercises and your back's the thing that's holding you back. A belt actually helps you develop a better sense of how to use your ab muscles to support your spine & not just a way to get out of using proper form. For those of us who've got a solid technique down, wearing a belt can help you transfer force more effectively and means you won't break down into bad form when you're exhausted. That's usually the point at which progress starts to stall without some extra support. To be honest though I'd advise beginners to leave the belts alone. Once you've got your technique sorted and you're lifting some serious weight, a belt becomes a tool to help you lift better not a safety net because you're not lifting properly.
A weightlifting belt makes the biggest positive difference when loads approach a lifter's technical limit and bracing, not strength, becomes the bottleneck. In practice, this is heavy compound lifts like squats and deadlifts at roughly 80 percent and above, where maintaining spinal stiffness under fatigue matters most. The belt does not replace core strength. It amplifies it by giving the lifter something to brace against, increasing intra-abdominal pressure and stability. Used correctly, it improves force transfer and consistency rep to rep. The mistake is wearing a belt too early. It is a performance tool for maximal or near-maximal efforts, not a crutch for warmups or light volume. Albert Richer, Founder, WhatAreTheBest.com.