Read @Richard Cheese's post, phasing used to change zones. With current wording, we execute the old zone change so perfectly that we don't need to have a zone for it anymore because the rules tell us exactly how to do it without telling us what it is we're doing. When talking about phasing we have to be on the same page - no rules update has ever changed how phasing actually works, we silently omitted the term "phasing zone" by forcing permanents to act like they made an actual zone change with very specialized wording to describe the phenomenon of going from Zone A to Zone A (without saying anything about Zone A, but instead calling it a status effect).

Maybe an easier example is Chimeric Idol - if it is animated to a 3/3 and then phased-out, it is now a non-Creature Artifact. Now Time and Tide can't bring it back (it's not a creature when phased-out. No really, call a judge, you need to). If however Time and Tide instructed us to bring back non-Creature Artifacts that same turn, then Chimeric Idol would phase in as a 3/3 Artifact Creature. That's a smoking gun; a zone change (or something mechanically identical) just happened between that card being phased-in and phased-out.

Not knowing that interaction won't actually keep you from being able to play and understand magic. Not seeing the evidence and not conceptualizing the "battlefield zone" actually being two half-zones is totally fine, but when we say "it's ok, tokens don't have to exile due to phasing out" all that you are doing is poking holes in that rules wall that keeps the phased-out part of the battlefield from being something that can affect you. No one wants the phased-out battlefield to become active. All the mechanics say it's there, all you have to do look at the signs. Chimeric can't make itself a creature, phase-out into a noncreature, and [if it were possible to phase back in on that same turn without card type creature] come back as the same 3/3 creature - that's the same object exhibiting vastly different characteristics unbidden when in the same zone, something profound happened: we ran the card right through a zone boundary, and we didn't even know it!

It is easier to teach magic when we don't have to tell a player "this is the battlefield, it has a phased-in and phased-out part." That's why we have all these nutty rules that make phasing work like a zone boundary crossing, and why we can stop talking about the phasing zone and pretend everything is fine, it's just a simple status effect like tapping and untapping. We honor all the rules of a trip to the phasing zone even today - but that will stop once tokens pick up a random rule. This change is the first step in degrading the whole rules contraption that allowed people to go around thinking the battlefield is just the phased-in part and phasing is a simple status effect.

If the phasing zone is gone, why are we doing everything it told us to do when it was around? The answer is semantics, we just want to be able to clearly say stuff like rule 702.25d. When we do that it means that we don't have to educate people about a zone they'll never need to know, but the functionality of zone change is still all there though; this never changed mechanically speaking. Go find that judge and ask them what the animated Chimeric Idol or Memnarch'd March of the Machines are when they phase-out; you have to know that answer to understand how silly it is to see a card run so clearly through a zone boundary and claim nothing zone change'y has happened.

To conceptualize the problem: if that Chimeric Idol remained a 3/3 Artifact Creature in phased-out status, we are basically saying anything that it pulled into an indirect phase would also keep modifying it. If it ever gets to that point you have phased-out cards acknowledging the existence of phased-out cards - that's an entire second battlefield. It would totally destroy any mechanical way for us to keep talking about phasing as a status effect; we just went straight to everyone needing to know that the battlefield zone is comprised of two half-zones across all formats, to almost everyone's mutual detriment. No longer would we get to write a rule that says "there's no zone change, just do all the zone change'y things if you do this (as if the zone still exists)."