For the experienced, any of the alternate methods mentioned above will get you there. But they are all just an approximation of getting it right. A smoke trail only tells you that you aren't burning up all of your oil in the combustion process. That's good enough to know if your engine is safe, but it doesn't tell you if you have the mix set for max power. The ear method or the pinch method will get you close, but you don't actually know how close. With a tach, you can see the RPM rise with each click of the needle valve, then stop rising, then start to drop as you get too lean. It will guide you quickly to the exact spot where the engine is at the optimum mix, then allow you to richen it up for flight precisely. The amount it needs to be richened depends on how much the engine unloads in flight, but 400 RPM is a good starting point. Setting the idle is the same; get your lowest reliable setting and then watch what changes happen on the tach.
Besides making the process easier and more precise, the other benefit is repeatability. Engines need to be adjusted for weather, so if you know you engine needs to be 300 RPM rich to run right, it's a 1 minute process to find the peak setting and then richen it to where it needs to be. You'll get it right the first time every t l time once you know what your engine needs.