I find these structures absolutely entrancing, which is why I expend so much time and effort photographing them. To answer your question, in the 1970's they numbered in the thousands, to be found at every prairie town through which the railroads passed. Now, there are only several hundred left, most of which are in Saskatchewan and Alberta in Canada.
When they were built, they were utilitarian in nature: function was the prime consideration. But now, if you are lucky enough to stand in one these giants, with the structure creaking and vibrating to the ever present prairie wind, in an abandoned town in the middle of nowhere, well, you will know why I travel thousands of miles to photograph them.