A weightlifting belt primarily supports your abs, not (directly) your back. It sounds back, but here's why: the belt acts like a second set of abs to prepare your entire body to lift heavy loads.
To brace yourself for those super heavy lifts you've taken a deep belly breath and hold it, a method of “breathing” called the Valsalva maneuver. The Valsalva maneuver helps create intra-abdominal pressure that cushions and supports your spine. And that's where a weightlifting belt bestows its powers. With a lifting belt, you do your deep belly breath into the belt, which pushes back against your abs. This amplify the effects of that intra-abdominal pressure, and in turn, helps protect your back and lets it handle the stress of heavier loads even better. Weightlifting straps are a sure way to fix your bar in place and prevent sliding.
Weaving a belt by itself will not automatically level up your strength and lifting ability. There's a learning curve to wearing it and lifting with it on (just like there's a learning curve to being able to properly apply intra-abdominal pressure and lift). Sure, some can reap the benefits right away, but it'll take most a while before things will click.
When you throw on a belt and use it properly, the skies part, birds sing, and your deadlifts or squats (or both) get a noticeable boost. Greg Nuckols of Strongtheory.com found that well-trained belt users can generally move 5-15% more weight for the same sets and reps, be able to squeeze in an extra couple reps at the same weight, or lift the same weight for the same number of reps with less effort. That's pretty significant!
We can take this to suggest that over time training with a belt will likely get you stronger than training without a belt. This makes sense in the context of being able to do more overall “work” (ie lifting more weight and banging out more reps) and continuously push your body to improve, a process called progressive overload. In the long-term, you can gain more muscle size and strength.
A weightlifting belt can be used for squats, jerks and deadlifts. Experienced lifters throw the belt on for near-maximum efforts, and take it off for regular training and warm-ups. Typically, “near-maximum” is a weight that is 80% or more of your maximum lift. The exact percentage is often arbitrary, so wear it when you think you really need the extra support on big lifts.
How to wear a weightlifting belt?
1) Take a breath (hold it)
2) Place the belt in position and brace the abdominal wall
3) Draw the belt just tight enough to slightly restrict your braced abdominal position to achieve maximum benefit
Weightlifting belts are going to be uncomfortable for a while, especially as you're learning to get used to one. However, once you get comfortable with your belt, you can start to experiment with changing the belt's position on your torso. For instance, Omar Isuf, a strength and performance coach, says that experienced belt-users tend to wear their belts higher on their torso during a deadlift than they do during a squat. More specifically, in a deadlift you might find wearing it around the mid stomach to be more comfortable. During a squat, you might like it above the iliac crest.
You also want your belt to be tight enough to stay in the same place while you lift, but not so tight that you're cutting off full, deep breaths or circulation. The clothes you wear and even down to how much water you're holding could influence how tightly you have to pull your belt. If you're just learning, it's okay to wear it a tad looser until you learn to love its not-so-tender embrace. Finally, keep in mind that your new belt needs to be broken in the same way a shoe does.