It's pure optimization, not really mandatory.

Ideally you want to engage with ~5 seconds left on ninjutsu cd. You want to do a dancing edge combo while weaving in your offensive buffs (and potion if you REALLY want optimal burst), then suiton->spinning edge->TA->shadow fang->DWD+mug->mutilate->kassatsu->spinning->raiton->gust->(jug if on stun-immune boss where it has no other use to save for)->duality->AE. [TA expires here]. It's also ideal because that's usually the mark where mechanics begin, so having the initial burst already done helps.