I have to chime in here as well. With out a doubt the best tuning tool of the past 7 or 8 years when they became affordable. (ie inovate motorsports). For all the reasons others here have stated.

Here is another reason others have not noted, but I would like to address.

The ability to determine weather you have an ignition or fuel related problem. Sometimes the differences can be subtle. The wide band will tell you right away if you have an ignition problem.

I have tti headers that run very close to a plug wire. The wide band will tell me if the wire is compromised, before I even get a hint of a miss while seat of the pants tuning.

To be very honest, I lack the necessary carb tuning skills to even guess at the proper carb set up without a wide band. Because of the way modern fuels are today , don't tell me you can "read plugs" with a flash light and magnifying glass. You can use a magnifying glass, I will use a wide band.

While if your budget allows, a dyno and a stopwatch are very hard to beat as the king of tuning tools.