I just devoted roughly five minutes of internet research to this subject and didn't find anything even remotely close to an explanation, so clearly the answer to my question isn't obvious. What is the connection between banks and large digital clocks/thermometers? Who started the trend? For what reason? Why did it catch on?
I'm guessing it has to do with banks as a source of information, but that's only a half-baked theory and I know I'm missing something.
Also, why did it take me 30-odd years to first wonder about this?