My camera has a light meter, can’t I just use that?

Yes and No. Your camera’s meter is a reflective meter. This measures the light that reflects off of the subject. The Illuminati is an incident light meter. This type of meter will measure the light source or ambient lighting.

While there is cross over in uses, an incident meter can allow you to set ambient exposure and intensity of light that you want to illuminate your subject. An incident meter allows you to precisely measure the light independent of the influence of the subject which the light falls.