There are a wide variety of nipples, used for a wide variety of things. Exhaust gas recirculation for instance (EGR valving). EGR is best avoided when in the company of others... Now some nipples are as worthless as the tits on a bore, while others are much more interesting, but I digress...
In one case I used a 'spare' nipple to rig up a long vacuum line to a vacuum gauge in the cockpit, so I could monitor my performance whilst motoring along. If you know what the vacuum rating of your carbs 'power valve' (PV) is you can ascertain when it opens under varying throttle conditions and correlate that to how the car 'feels' at that instant. As well as how the vacuum advance may be impacting performance.
Reading a vacuum gauge is an 'art', not an exact science. Knowing what nipples to use, how and when, is also an 'art'. I do consider myself an expert in the proper application of 'nipplizing' techniques.
