Use the tach to measure the rpm. then lean (clockwise) the HS needle valve by a few clicks - about 1/8 turn. check the rpm again. keep doing this until you find the maximum rpm. Then open the needle valve until it slows down a few hundred rpm. Now you have used the tach to properly set the veedle valse. Most of us can also do this "by ear", but the tach makes it more scientific and more fun. Other uses are to compare things. For example, you want to compare your fuel to your buddies fuel. You tach the engine running your fuel (suppose it has 10% nitromethane). Then you tach it with everything the same (same prop, etc.) but using your buddies fuel (suppose it has 15% nitro). Now you can determine if the extra nitro makes any difference. (usually this will increase the rpm by a few hundred, by the way).