It's to give you the tiniest bit of flexibility in how you use your spells for mobility. You might want to delay Ifrit's Gemshine/Brilliance if you can't afford to stand still for an extra 1.3s per cast right this moment, or you might want to hold Titan or Garuda's Gemshine/Brilliance because you'll need unrestricted movement in a few seconds. In each minute you have time for 1 demi summon, 6 boosted Ruin IIIs, all your Primal summons and all their spells, one Ruin IV, and one unbuffed Ruin III. That one unbuffed Ruin III can be used to shift your mobility by one GCD so that you don't run out of mobility in the last second of a long move.