Once upon a time, back in the days of film, long exposures (like of the picture below, which was taken at 2:00 AM using a 20-minute exposure) were very much a trial-and-error process. First you had to put your camera on a tripod, and set your shutter speed to “a very long time”. It was difficult to know how long to keep the shutter open, as the light was too low for the camera's meter to measure accurately. So it was a trial and error process. Standing out in the cold monitoring your camera and the elapsed time was an exercise in tedium.
What to do? I solved the problem back in college by hooking up my camera to a Hewlett Packard 41C calculator (really, it was a computer – thankfully none of my professors knew that), building some custom hardware, and programming the calculator to try different exposure values based on an initial time. So if I put in a base time of "1 minute", it would take six sequential pictures, one each at 1 minute, 2 minutes, 4 minutes, 8 minutes, 16 minutes, and 32 minutes (one stop apart). When I got home and looked at my slides, one of those exposures was sure to be the right one. Here's one example where computers could do all the work for me while I stayed warm inside, sipping hot chocolate.
My calculator was always in my camera bag. Not only could it control my camera, but I programmed it to tell me the sunrise and sunset times based on my geographic coordinates.