It keeps a shaded panel (or series-array in this case) from pulling down the output from another array wired in parallel. Multiple parallel connections to the same controller need to be isolated from each other. The voltage drop from a blocking diode is about 0.4 volts and while that's a slight loss of efficiency it's not worth the cost of a second (or three more) controller(s) to alleviate it. Also I suspect the 0.4 voltage drop is less of an issue at 34 or 68 volts than it is at 17volts.