Why dose....

These elements aren't really the normal... Although one can definitely test for them.. I know potassium and Iodine and perhaps even strontium can enhance color... But i would make sure the main 3 are in order before you dive into those... Last meeting was an excellent example of what these items do. Although i cant remember off the top of my head. 

For the vast majority of reefkeepers, a good salt will do 99% of the work. As things grow, they need addition of alkalinity, calcium, and magnesium, so that takes dosing (in form of 2-part solution, calcium reactors, kalkwasser dosing, etc.)

There are very few reefkeepers who *need* to dose potassium, iodine, iron, manganese, strontium, etc. as long as they are using a good salt with regular water changes.

To be honest with you, testing for those additional elements can be a pain...especially since tests like potassium are an absolute pain. I find that testing when I am not happy with my colors is the best option. Typically, if you aren't happy with your colors, you'll find other issues as the culprit (too much carbon/GFO, too high nutrients, etc.)

You're much more likely to overdose those elements than to suffer a detrimentally low condition. I've suffered a low potassium issue once...and that was an obvious issue looking back...even without a test kit. I've never had an issue with low iron, etc.

Potassium will affect the tissue health of Seriatoporas/Stylophoras/Pocciloporas, and a lack of it will eventually cause a loss of blues in Acroporas.

Strontium is known to replace calcium in coral skeletons in low calcium environments, but I don't think it's know exactly how critical it is in calcium-building.

Iodine (and its close relatives-iodide, etc.) help give protection against UV, helps invertebrates build skeletons, etc. It's pretty important, but it's also nearly impossible to keep levels at NSW levels. Most reefkeepers just let it be with no deteriment.

Um...not sure I answered your question, but I hope that helped!