Hahaha! I registered JUST to respond to the comments made by Az.

I've tuned home and car audio systems for many years, by ear and by SPL meters and spectrum analyzers. I understand your point, Az, about the variables. But let's try this example:

Have you ever had your wheels aligned? The adjustments are very simple. You could even do it by eyeballing. And yet any respectable shop will do it by laser and computer. Why? What about the variables of the car, like tire wear and pressure, weight of the driver, full gas tank or half full? How about the rotational mass of the wheels? Road surface imperfections, wind resistance, humidity...it goes on and on. All of which can cause your car to drift after a perfect alignment, so you turn the steering wheel to correct it. Using an SPL meter is no different.

You use the equipment to set the baseline of what a reference point SHOULD be. There may come a time when you change it to what you WANT it to be. Sure, if your car pulls hard to the right you just crank the wheel to the left and it goes straight. But wouldn't it be better if it just went straight to begin with? If you want to align a car by eyeballing, you could do it - it might even go straight, after a few dozen attempts. But it'd be a helluva lot easier just to use the darn equipment!