Lever Action Belt Buckle Placement
As everyone has stated, it is an individual preference.
The overall objective of the belt is enable you to push/pull more weight via providing intra abdominal pressure, IAP.
This is accomplished by isometrically contracting the abdominal muscle into the belt.
This isometric contraction provide more support for your back, core. Greater core stability translates to pushing/pulling more weight.
Let's expand this as to why...
Dr Stuart McGill's (Professor of Spine Biomechanics at the University of Waterloo) research has demonstrated core stiffness is a major determinate in the amount of weight you are able to push/pull.
The greater core stiffness, the more you lift.,
If you core is weak, "Leakage" occurs. That meaning the core is more limp. Energy (Strength and Power) are lost. Thus, you lift less.
Increasing Ab/Core Strength
The belt allows you to isometrically contract your abdominal muscle into the belt promoting greater core stiffness.
The stronger your abdominal muscle are, the greater your abdominal isometric contraction is, the more Strength and Power you generate.
With that said, additional Ab/Core Strength increases your Strength and Power. Additional Ab Training is mandatory for Long Torso lifters.
The Torso is "The Bridge" that connects the Lower and Upper Body. As with all bridges, the longer the bridge the more support it need.
...makes a good point, "I use different belts for different lifts." It is addressed in...
The Belt And The Deadlift
As Rippetoe points out, a 4 inch (10 cm) Belt for some individual impair their Deadlift Technique.
As the article noted, there are two solutions...
1) "... 2-3 inches wide" Belt that is tapered in the front.
2) "... A much higher position, well off the hips" with a 4 inch (10 cm) Belt, as demonstrated in the.Konstantin Konstantinovs video attached.
The higher belt position often is more effective for Conventional Deadlifters who have some Thoracic/Upper Back Rounding.
Upper Back Deadlifters intuitively use this method to pull more weight. Upper Back Rounding enables lifter to decrease Torque (which magnifies the bar weight beyond it true load) by keeping the weight closer to the Body's Center of Gravity, COG.
Upper Back Rounding is okay. Lower Back Rounding is counter productive.