You can tell who doesn't do real endgame content with all the complaining.

Mechanical buffs are prioritized left. Whether you need to know who on the party list you need to swap or join for certain mechanics. Of course there was a savage mechanic where you had to read right-to-left.

Whether it's being Astro and knowing that certain buffs are up helps where to prioritize who to give cards. FoF, Tethers, Dance buffs, etc.

This also true for Black Mage and MP bar can definitely show where they are in the rotation.

As a healer, if the Dark Knight is severely low on HP they might not be able to TBN and therefore will have less or no mitigation for certain mechanics.

MP used to be renewable resource from other jobs like casters and phys ranged back in Stormblood. Things have changed now.

But the list can go on and on...