A vacuum gauge is pretty handy as a diagnosis tool but otherwise it will normally just wander between 20 and 5 inches of mercury most of its life as you accelerate and coast. Don't knwo why they settled on a 40 inHg scale. You'll never even touch 30.
The more head scratching thing is the package includes an analog clock, but if you already have the digital clock in the dash there's not a lot of purpose to it besides it just whirrs away constantly.
One thing that the harnes DOES help with in that matter is it adds the tachometer wiring which goes in the place of the digital clock. I have a salvaged one but they are apparently prone to failure. Mine on inspection seems that the adhesive that fastens the needle to the coil has failed and the needle is decoupled from the actual gauge. I'll need to work on that at some point but installation will require pulling most of the dash trim and the cluster out.