On my 2003 Sierra SLE that button was for the magnetic ride control which kept the vehicle level under heavy loads or bumpy terrair. If you look under your truck, you should find orange Delphi shock absorbers with a wire going to them. Turning the magnetic ride control on actually makes the ride rougher but helps with level cornering. Oh and those orange Delphi shocks? $400+ each from a dealership.
Pros: helps with level cornering
Cons: makes ride rougher, high cost
