Like Shield. I'd probably set Shield to trigger automatic whenever my wizard would be hit and it would save him from taking damage, at least at lower levels when HP is low. I don't need a prompt at that point because casting Shield would save his life. I'm going to cast it every time because he has low HP. Later, I might turn it off or set it to prompt because he has more HP and I might not want to waste the slot.
This will use your spell slot. sometimes it's better to take the hit
Yeah, I know sometimes it's better to take the hit. The point is that presets can manage it for you. Are you sure you don't want the spell to be cast because you really don't want it to use your spell slot and you'd rather take the hit? Turn it off on your turn. Maybe you're facing goblins and such who do minimal damage. Yeah. Turn it off. You don't need to waste the slot.
But what if you're fighting phase spiders and you REALLY don't want to get hit by them. No question. You REALLY don't want to get hit. Turn on Shield, and if a phase spider rolls well enough to hit you but not well enough so that if you cast Shield it'll miss, boom. Spell triggers automatically. No prompt needed.
But what if you're just not sure? What if you don't know whether in this particular battle you will want to cast Shield or not? Turn on Prompt. Now, any time someone hits you, a prompt appears and pauses combat.
Three simple presets:
On = Yes. I'm absolutely sure that in this battle I will ALWAYS want my Reaction to trigger as soon as someone triggers it.
Off = No. I'm absolutely sure that in this battle I will NEVER want my Reaction to trigger as soon as someone triggers it.
Prompt = Hmmm. Maybe. I don't know, so the game had better prompt me because I'm just not sure.
What's the point of these 3 options? Those who don't like combat being constantly interrupted by reactions they're just going to say Absolutely Yes or Absolutely No to don't have to have combat interrupted by prompts if they don't want to. However, if you're the kind who wants prompts every time, you can turn on prompts for your reactions and have them trigger every single time. AND if you're like me, you may want the best of both worlds. You may want some reactions to trigger automatically while others trigger with prompts.
