I came up with this analogy: Imagine that film is like a bucket, and light is like water. Normally, when you click the shutter, the right amount of water flows into the bucket. When the light is low, the water trickles or drips into the bucket, and its rate is so slow that some of that water evaporates out. So to get to the proper "full" mark, it takes more time to fill the bucket.
Originally Posted by MattKing
(And for making photographs like Michael Kenna, scene selection is important. One time I photographed a landscape by moonlight, and Kodak E100S looked like daylight after 15 minute exposure at f/2.8. I'm sure Mr. Kenna made lots of test exposures to get the appropriate reciprocity data for his needs.)