Typically boost gauges are in Turbos more than Supercharged cars. They are easier and more likely to be altered and running high boost, and need a gauge. Most turbo setups can be setup to override factory computer controlled boost with a simple mechanical boost controller for about $12, and you have "Dial a HP". I used to do that on my old Chrysler 2.2 turbo Shelby Chargers and my Turbo Rampage. But mainly, they are just for looks for most people.