A turbo- or supercharger are both engine driven air compressors and provide varying levels of boost according to engine rpm, load etc. Quite often there is a powerband within a given range of available boost pressure and it is an aid to performance driving to be aware of when that powerband is being approached, in the same way a driver wants to be aware of engine rpm.
A boost gauge is mandatory when boost pressure is being modified to higher levels than OEM standard on a production turbocharged car. Simple methods can be employed to increase factory boost levels such as bleeding air off the wastegate diaphragm to 'fool' it into staying open longer. To avoid excessive leaning out of the engine (caused by increasing the boost beyond the fuel systems capacity) care must be taken to monitor boost pressure levels, along with o2 sensor levels.