The one thing a tach will do that's never mentioned, is that you can learn to set a needle by sound with it and not need it later. It will tell you what that sound MEANS. The sound alone won't tell you squat if you're a newbie.

It's a good learning tool.

Use it awhile and you'll learn ACCURATELY what pitch to listen for when setting your favorite engine. Make sure to use it on one engine almost every time and it won't take long until you can hear the right rpm.

It's also a good tool to use to find out just how much more rpm you're getting from changes in fuel or prop. It's worthwhile to know exactly what the difference is in the change in rpm. Anyone can hear faster or slower, but nobody can hear "how much". With a tach, you ain't guessing.