There is research suggesting that astaxanthin promotes accumulation of zeaxanthin in the retina. Is that at least one reason that inclusion of astaxanthin would be beneficial? What else is important for us to know?

That is one of the properties that makes it advantageous. There are others as well, still unappreciated by medical science. So astaxanthin as a standalone is quite beneficial in a supplement for helping eye health in general. It also works well when combined with other similar chemical constituents because that is the intent of those sources in nature that contain multiple substances within this chemical family. Some scientists would dismiss the value as representing chemical intermediates, where the primary end product would be the best choice, and they would assiduously work to purify that perceived final peak member of the chemical family present so as to capture the most potent of the individual molecules. But we can tell you that the variety of chemical constituents does not represent a gradual processing and metabolic conversion to inactive derivatives but actually, for the most part, represents an array of chemical substances designed by us to work well together as medicinal agents, and in some instances having benefits for the plants themselves as a double benefit to the planet. Overall, we can tell you that in the majority of cases, looking for the premier best constituent among a chemical family is a fool's errand when a mix of similar substances naturally present will usually perform better in achieving a chemical enhancement of something useful in the human body.