Hmmm.... IMO, although its a novelty factor and quite cool to see clear downtubes (which turns white anyway because of the foam bubbles and no diff from white PVC pipes)... it has zero practical purposes.
You monitor the quality of the bubbles in the main reaction chamber & riser tube coz it's impossible to see them rushing by at high speeds in the downtube.
Previously I had to wrap my old skimmer's body in paper to prevent diatoms, slime algae, coralline algae from growing on the body! Light will trigger growth of such algae thru clear acrylic!