Superthumper:

For tuning your carb.....the O2 sensor and gauge is all you need to have, and you can adjust the jets and needle to make the most HP, run the smoothest, get the most mileage....by adjusting the mixture and knowing which way you need to change things for improvement.

If you want the most HP and a way to measure it as you change engine and exhaust parts, mufflers, headers, cams, etc.....then the DYNO time will show how things have changed and what adds HP and what doesn't.  The DYNO also will also put numbers to what is happening....and provide a more accurate record than the seat of the pants testing can.