its simple, its to allow you to troll a much greater area as well as allowing you to troll with multiple lines.

lets say you have 4 people in the boat and each has two lines. having 8 lines going straight back off the boat will be very difficult to keep them from crossing lines. with planer boards you can send 1 out each way and have the lines go down them with these clips that will release when you get a bite (also releases if wind or wave is too strong or if you get it into weeds). using the planer boards will allow you to have the line way out past the edge of the boat and are very convient if you are trolling multiple lines.