David Feldman in Why do Clocks Run Clockwise? posits that 1) the boots may deter coyotes. 2) It's better to stink up a fencepost than a house. 3) Some people just do things.
But forget about the boots. Why do people put caps (coffee cans, finials, can ends, and hub caps) on fenceposts? Because less water (and snow) on the ends means that the posts don't soak throughout and last just a little longer. Then again, the third explanation above also makes sense.