I guess you just have more faith in the data the weather app on the iPhone throws up.
I find the single day temperature / image to be too much of an average (so 12 degrees and sunny means that it's between 6 degrees and 18 degrees which basically doesn't help me for that sort of decision making).
Maybe that's just where I live and that for others it does provide useful information.
Or work in an office without a window? Or live somewhere where it can be sunny outside, but still need to know if you need to take a jacket to lunch?