It makes most sense to use a boost gauge if you mod your car in such a way (i.e. chipping) that the stock levels of boost will be altered. This is for two reasons. 1.) It tells you exactly how much boost you're pushing and when (don't want to push too much) 2.) You can see if boost levels are abnormally low, which might mean a leak somewhere. The second reason could be used whether or not you have mods.