That explains the need for a waxy layer, but it doesn't explain the need for the superhydrophobic bed-of-needles surface.

Two possibilities:

One, a superhydrophobic surface reduces the surface area of the water that lands on the plant, allowing less of it to evaporate and more of it to fall to the ground.

Two, it may help prevent plant diseases by allowing less water to attach to surfaces that are prone to becoming diseased.

That was my first thought. It's not a good idea to water your plants at night, because without the sunlight and heat of the day the water doesn't evaporate as quickly. Water sitting on the leaves all night can cause fungus and such things to grow, which is bad for the plant. Since plants can't avoid being rained on at night, it seems logical that they would evolve a mechanism for staying as dry as possible.

Leaves blow around in the wind. If water was not being repelled from the underneath of the leaf, it would gain added weight, and have the potential of anchoring the leaf in the wrong direction. Repelling water from the underneath of the leaf keeps the leaf light, which allows the rest of the plant structure to support the leaf in the proper orientation.

My first guess would be that it requires less energy to produce and maintain than other similarly effective arrangements of the wax.