ST / Core Caster
You bring these when the game throws heavy defenders at you that laugh at your snipers' attacks. You can get by with just one. Note that for units without high DEF, you'd rather bring a sniper for more damage, so casters that prioritize high DEF enemies (Ceobe when using s2, Steward's talent) are very valuable (as those are the enemies you want to use casters against anyway).
Generic but strong options for ranged Arts damage that can fit in any sort of team. Usually paired with strong burst damage options, very useful to take down a plethora of armored enemies.