[Delphinus]

I consider it more a guideline rooted in "best practises."

It is not a magic number that poof, after 6 months your tank is ready for say anemones. But a person setting up a new tank might be gung ho to add anemones or other really sensitive critters (and remember the definition for "sensitive" in this context really means "they die easily and sometimes for no apparent reason") it is better that they give their tank some time to truly establish enough biological capacity towards resiliency because there are a lot of things going on that you don't always see and this can increase the risk towards some of those creatures. So by advocating a wait for 6 months you're hedging the bets in favour of more success. Also if the aquarist in question is a newby they can use that time to learn and padawan themselves up in the meantime and honestly that journey in itself can be a lot of fun (I just love reading about how someone new to the hobby talks about how awesome it is to look in a tank and then be surprised by some kind of new copepod or whatever that they hadn't seen before.).

Anyhow in my opinion though a tank hasn't really settled into its mature character until the 6 month mark. Doesn't mean I woudn't keep stuff in it before then but I do expect to see the odd algal bloom here or cyano there or diatoms ... it's like each "thing" needs to have its moment and crest, and once all those are done the tank is in a more steady-state situation and you can have a little more predictability that way.