You will only need one if you have done extensive modifications such as increasing the rev limit mechanically (engine mods) and electronically (reprogrammed ECU, etc) and need to visually quantify the engine speed and get a good shift light to boot.

Most of the time when driving on the street you won't need that, but if you take it to the track then yes you will benefit from it if properly done.

Other than that, it becomes too tach...err...tacky