I never ran one in my tank. I guess it is preferred to get a quick visual indication of how much co2 is in your system. I personally prefer less stuff in my tank. You can calculate the CO2 levels by measuring your pH and KH. It is not accurate if you have buffers in your water though.
Source: www.petfish.net
Edit: After looking at the chart, I would say 30ppm should be yellow, not 25. I ran mine as close to 30ppm as I could get.