why use rotating racking arm?

Why would someone with a conical dump valve need a rotating racking arm for sediment-free transfers?

All I can of think is that perhaps there is some settling that sticks to the sides of the tank. I wonder if some type of slight vibrations would increase the settling?

I have a rotating racking arm on my conical, and I've never needed to rotate it. I just leave it in the downward position and it racks fine.

The only reason I could think to rotate it might be if you had an excessive amount of trub settled at the bottom, and you wanted to move the pickup above the trub line.

Flipping the racking arm can be useful if you want to krausen a sluggish fermentation and mix up the fermenting beer (and not disturb the settled yeast in the cone) with more active yeast from another fermenting beer you have to help you achieve your target final Plato. That or you can just add some fresh yeast of the same strain in the same fashion. It can also be left in the normal position to help with yeasts that are more thick and like to stick to the sides of the cone if you are used to just racking off the bottom port. Less yeast will go through during your transfer after you dump the preliminary yeast that the racking arm is sitting in.