Dump valves are designed to vent unused pressure to atmosphere when the throttle has been released. When accelerating the turbo charger produces positive pressure which is used to supply the engine with large amounts of compressed air. When you lift the accelerator the throttle butterfly is closed and the boost has no escape route, so it backs up through the intercooler and causes back pressure that tries to prevent the turbo from spinning, this is known as compression stall.
By fitting a dump valve every time you release the throttle the excess boost pressure will be released which means a significant reduction in turbo 'lag', contributing towards faster pick up between gear changes. In extreme cases on cars with high revving engines or large turbochargers the shaft within the turbo has been known to snap when no form of dump valve has been used.