I finally figured it out --

I agree that it's useless for deciding what to wear before you get on the bike, but it's useful if you're already on the bike and are trying to decide if you should be wearing the same stuff next time. If, for example, you're ten miles into the trip and start thinking, "Man, my hands are just too damned cold," you could see what temperature it is and plan to wear warmer gloves the next time you see that temp on the forecast.