Because everyone has an opinion and at least 50% of them are wrong. Welcome to the Internet age.
All you can do is follow people with long term expertise and try to stay on track with all the background noise. Mr_X on this forum makes it as simple as it can possibly get and still work. I go for a more complex solution, but we both grow corals like crazy.
Many solutions work well together, but in order of importance, assuming water exchanges are first filtration priority I believe it's;
Sufficient live rock
Protein skimming (tanks over 20 gallons)
Then comes DOC
filtration that's missed by the first two, usually only needed if you want SPS
quality reef water and the primary filtration is missing that last little bit, or nitrates are building up;
or other P02 binding agents
Carbon for coral toxins if the reef is packed.
Then maybe other stuff like Purigen to soak up selenium and any DOCs the previous equipment missed.
You can use all if it, or a portion of it. Belt and suspenders approach. There is some overlapping, but that's a good thing.