A paper was published this month in Applied Animal Behaviour Science in which authors summarize the origins and various hypotheses about the function and utility of clicker training.
Full text of this article, How clicker training works: Comparing Reinforcing, Marking, and Bridging Hypotheses, by Lynna Feng, Tiffani Howell, & Pauleen Bennett is available here:
http://www.sciencedirect.com/science/article/pii/S0168159116301393