MSQ

[Iscah]

Exactly!

So why are they required for the MSQ?

Because the roulette labelled "Main Scenario" is an odd labelling choice that is more accurately the "ARR Finale" roulette.

All the other dungeons that form part of the MSQ are included in either the levelling or 50/60/70 or level 80 roulettes. That does not somehow strip them of context and make them unnecessary for the MSQ.

The simplest answer is, this is a JRPG at its core with an MMO wrapped around it, and a JRPG generally involves progressing through a series of story-relevant dungeons. So that is what this game does. It tells a story designed to guide you from dungeon to dungeon with story events inbetween.