Using a good belt helps with keeping a braced trunk. The belt gives the abdominals a surface to push against, so when the lifter takes a breath and braces, the belt pushes back against the abdomen, increasing intra-abdominal pressure (https://www.ncbi.nlm.nih.gov/pubmed/2709981). Intra-abdominal pressure contributes to increased trunk tension and stabilization and possibly relieves compressive forces on the spinal discs (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2949466/).
A braced trunk with a lot of tension means that more force gets directed into the movement of the bar from the legs or arms, and less force is lost through a back or hip position that loosens or rounds throughout the course of a lift.
Good trunk tension also makes sure you're using the right muscles. If the abdomen isn't braced correctly, the lifter's hips may tilt behind them, reducing the ability of the glutes to extend the hip. This would have obvious affects in things like locking out a deadlift or driving out of the hole of a squat, and it would even affect the stability of the trunk during an overhead press.
Our very own Coach Chris Riley wrote his master's thesis on the effects of the use of a belt on the 1rm squat of trained lifters. You can read the paper below, but to sum it up, all participants in the study were able to perform a heavier 1rm squat with a belt. The participants were all "trained," that is, not new to lifting weights.
Will Using a Belt Make My Abs Weaker?
And while squatting, deadlifting, and overhead pressing all train the abdominal muscles, it's also important to actually train the muscles of the trunk. Both belted and un-belted lifting benefits from training the trunk muscles directly, and planks, side planks, back extensions, and abdominal flexion should be a regular part of every lifter's program.
What constitutes a good belt?
Smaller people, or those with shorter torsos, may be more comfortable in a 3" belt, or even a 2.5" tapered belt.
The belt's function is to provide a surface against which the abdomen can exert pressure against, so the wider the better. Tapered belts won't be as effective and should only be used if a lifter feels that the ribs are being pinched or compressed as they move with the belt.
Thicker belts will obviously provide more resistance which feels great when descending into a squat, but may make getting into position to start the deadlift difficult.
Stiff leather belts provide enough resistance to significantly increase IAP. Thin nylon belts and soft leather belts (which are usually thin and tapered) are also made, and while these aren't as useful for heavy squats and deadlifts, they are better suited for lifts like the snatch and clean and jerk since these movements involve quick transitions in body position that may be hampered by a stiffer belt.
Where Can I get A good leather Belt?
Rogue, Inzer, and Titan all sell durable belts as well. Rogue probably has the most readily-available belt, Inzer ships their belts in black fairly quickly, and Titan has several options available to look at.