To balance carburetor and mechanical fuel injection air intake volume you have to be precise. Other methods are close but you need to be 100% accurate for maximum performance.
If each cylinder doesn't have exactly the same air intake volume, the horsepower produced by each cylinder will vary causing the loss of power and shortening engine life.
Capillary attraction of a fluid in a glass tube creates a meniscus (a concave curve for most liquids). A Manometer is so accurate you can place the bottom of the meniscus curve on the top edges of the scale line to make precision air intake adjustments.
The Manometer also works with side draft and up draft carburetors.
Back in the 1950's, 60s, and early 70s, Porsche required each of their Dealer Service Departments to have a Manometer to tune carburetor and mechanical fuel injection engines. Porsche realized that a Manometer is the only tool that will produce the accuracy required to calibrate air volume with adjustable throttle plates and have the sensitivity to calibrate air bypass screws for non adjustable throttle plates.
Think of the time and effort you have already spent to gain maximum performance from your engine - how can you leave each cylinder's air intake volume to chance by not using the most accurate tool available?