I'm new to reefing so take all of this as based on science not reefing experience...
I expect that most reefers are adding iron without even realising it. Fish food will contain iron (just as human food does, animal and vegetable). Uneaten fish food which is broken down by bacteria will release traces of iron into the water column. It will be quickly utilised by algee or sediment/skim out.
Much like your tank inhabitants are constantly creating ammonia but it is being processed simultaneously so doesn't show in tests. I expect a similar process is occurring with iron in your tank. It's there but fleeting and not detectable.
If you dose lots of iron it will probably help your algee process nitrates better but too much iron runs a small risk of causing iron overload in your fish.
What is your main reason for dosing iron?
The reason I'm dosing it is not specifically for iron, Metals+ is part of the Colombo colours dosing system. Basically a bunch of traces a bit like the RedSea Colors programme.
Long story short, everyone says "get nutrients low, light high and the big 3 stable and your corals will do well" ... well not in my tank they don't! They don't bleach or brown, the colours just fade away.
So I got an ICP test, and found some problems which I am now aiming to correct. One problem being that the elements present in Metals+ were all zero in the test results. Hence dosing this to see if it helps.