Pond windows allow you to inspect for infections where they typically begin - on the bottom.
Ulcers can develop around the pectoral or ventral fins and go unnoticed from a top-down perspective only. Catching symptoms early will help identify and treat problems sooner.
As a tip, sometimes it helps to record a video of the koi in the window. Then you can zoom in on concerns and provide a screenshot to your veterinarian.
Actual Customer Image
R. Higgins, Pond Portal Customer (Image Provided)