As question states, I can life a lot more on deadlift and squat by using a belt to brace against, why? Is it cheating?
I've often felt there were two aspects to using a weight belt. The first being the psychological sense of security that the belt provides. Belts make us feel “locked in” and ready to lift thus providing a positive framework to perform the lift.
The second and more important aspect is the potential support that a belt provides thus reducing the opportunity of injury when lifting heavy. Two separate studies: Effect of a stiff lifting belt on spine compression during lifting and Effects of abdominal belts on intra-abdominal pressure, intra-muscular pressure in the erector spinae muscles and myoelectrical activities of trunk muscles concluded that
“Wearing a tight and stiff back belt while inhaling before lifting reduces spine loading. This is caused by a moment generated by the belt rather than by the IAP. “
“Wearing abdominal belts raises intra-muscular pressure of the erector spinae muscles and appears to stiffen the trunk. Assuming that increased intra-muscular pressure of the erector spinae muscles stabilizes the lumbar spine, wearing abdominal belts may contribute to the stabilization during lifting exertions. “
So, belts may help prevent injury to the low back during heavy lifts and can increase performance from the stabilization they provide. On the negative side, however, using a belt may inhibit motor learning in the abdominal muscles as well as reducing the opportunity for the lower back to get stronger.
In short - belt gives better stabilization.
Why not to use it? Well, you wish to have great stabilization... You would like to use that muscles, not to support them.
Is it cheating - since that is legal - it is not cheating. Same as wrist/knee stabilization, special pants. All that helps you lift higher values - if that is your target? On the other hand why use hooks? If you can't handle that much - leave it. You need to find out your balance, do not judge others, just do your job and be the best! :)
Previous answers all missed a crucial aspect... the belt works by allowing you brace your abdominals against it. It doesn't "support" the back. It allows YOU to better support your midline/trunk/core/fad-name-for-that-area more effectively.
Generally, you should do as much work as you can without the belt (get used to generating that intra-abdominal force without something to push back on), but put it on when the reps get heavy (or the pauses in the bottom get long). That said, practicing with the belt, even at low weights, to get used to it, won't hurt either.
It's not about your erector spinae. Its about how you set-up your breathing and contractile force to keep those erector spinae safe.
To quote Mark Rippetoe (read everything he's ever produced, trust me):
First, the lifter’s belt provides a way to increase the effectiveness of the muscular contractions around the thoracic and abdominal cavities during a heavy lift. It works by giving the muscles around the spine something to contract against, so that they can produce a harder isometric contraction with the belt than they can without it. Like throwing a wiffle ball versus a baseball, or like learning to clean with a piece of PVC instead of a barbell, it is hard to produce force against little or no resistance. The belt provides this resistance to the trunk muscles, and the result is a harder contraction when the belt is worn.
Increasing intra-abdominal pressure is similar to inflating a balloon inside your abdominal cavity. The pressure inside the abdominal cavity pushes on the spine to support it from the inside, while the core muscles in the abdominal wall and lower back push on the spine from the outside. This inside and outside pressure acts to stabilize the spine and reduce the stress it receives when lifting heavy weights. This is how lifting belts can help to protect against back injuries during lifting. It's not due to the belt supplying the support, it's due to the way that the body reacts to the belt that supplies the spinal support.
References and suggested reading:
As others have mentioned it increases IAP. An effect of it is that it may also allow your core muscles to fire harder by giving your CNS a reason to reduce limits on certain muscles.
It probably isn't cheating
I disagree with the apparently common belief that wearing a belt will reduce the opportunity for the lower back to get stronger. Numerous studies have found no significant decrease in spinal erector activity when training with a belt compared to without, in actuality one study found an increase in muscle activity. Sources at end.
Add to that the anecdotal point that getting stronger with a belt also increases your beltless strength more or less directly, even if you don't train beltless during that time period.
Wear it. Most of the time
On the whole, wearing a belt increases your performance in the gym, which will help you achieve the goals of muscular size and strength. There are a few instances where it would be better to train beltless (if you train for a sport that doesn't depend on lifting maximal weights for example), but the bulk of your training should probably be done belted. It isn't cheating.
For a more detailed and highly recommended write up, refer to: http://strengtheory.com/the-belt-bible/
Studies about spinal erector activity with and without a belt: http://www.ncbi.nlm.nih.gov/pubmed/17047531 http://www.ncbi.nlm.nih.gov/pubmed/10619094 http://www.ncbi.nlm.nih.gov/pubmed/11932579 http://journals.lww.com/nsca-jscr/Abstract/1999/11000/The_Use_of_Lumbar_Supporting_Weight_Belts_While.14.aspx http://www.ncbi.nlm.nih.gov/pubmed/2304406