Some people macro fast cast gear at the beginning of their spell to reduce the casting time then macro into potency gear while the spell is casting so when the spell fires they receive the full benefit of the spell while still keeping the reduced casting time. This especially helps nukes where you need your nuking gear to gain the full benefit of the spell, yet that reduced casting time can make all the difference especially when soloing. If Quick Magic was a JT suddenly your nuke might fire off in your fast cast set which would reduce the potency of that nuke quite a bit.