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Rule 603.2 of the Comprehensive Rules says:

Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.

Lower, in 603.3 the Rules continue:

Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority

Why does it work this way? Is there a compelling reason why the ability can't immediately go on the stack? I know state-based actions will now happen before the ability is put on the stack, but I'm not sure why this would affect it.

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Magic is designed to avoid any interruptions to the resolution of a spell or ability

It might seem like placing triggers on the stack is a minor thing that would not cause any problems, but some triggers target and that means that now somebody is making decisions in the middle of the resolution of the spell. Even worse, some abilities trigger when the permanent becomes a target of a spell or ability, meaning that this interstitial trigger can trigger other triggers! (And one card, Scalelord Reckoner, targets when triggered by being targeted, which could cause it to trigger a third trigger!).

The rules are almost certainly capable of handling such complication, but it's not really beneficial to the game to have to do so.

Having all the triggers placed all at once reinforces the general intuition about how Magic treats simultaneity - how if multiple creatures enter the battlefield all at once, they all see each other enter for their triggers.

It also means that you don't have to figure out where various objects are at the exact moment the trigger happens when choosing targets. Everything waits until resolution is complete and everything is settled and stationary before target choice happens.

Additionally, if abilities entered the stack while a spell or ability was resolving, that would meant that you are now resolving an effect that is somewhere in the middle of the stack, which is philosophically not how the stack is meant to function.

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    "Fortunately, there do not appear to be any triggers that trigger on targeting that then target" Scalelord Reckoner does exactly that.
    – murgatroid99 ♦
    Commented Jun 3, 2020 at 1:11
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    @murgatroid99 oops, missed that one. Fortunately, that only enhances my point.
    – Arcanist Lupus
    Commented Jun 3, 2020 at 4:10
  • Also you could have Silverfur Partisan and Tolsimir, Friend to Wolves in play. The target would trigger to create a wolf which would then trigger to fight a target.
    – Tim B
    Commented Jun 3, 2020 at 13:51
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    @Tim_B, no, because the triggers still wouldn't resolve until later. Only things that happen when an ability enters the stack matter.
    – Arcanist Lupus
    Commented Jun 3, 2020 at 14:57
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    @doppelgreener Gargos and Lash only trigger when targeted by spells, so they aren't relevant.
    – Arcanist Lupus
    Commented Jun 4, 2020 at 12:34
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This is needed to enforce Active Player - Not Active Player ordering.

Imagine both players controlled a permanent which had a triggered ability "When a creature dies <something something something>" and a creature dies. Both abilities now trigger; without the two-step process in 603.2 and 603.3 the order in which the abilities go on the stack would not be defined as both abilities would go on the stack simultaneously. However, because 603.3 specifies the abilities go on the stack only "the next time a player would receive priority", the active player's ability goes on the stack first, then priority passes to the not active player and their ability goes on the stack.

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    I don't quite understand this... 603.3b says "If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose." If triggered abilities went on the stack right when they triggered, instead of the next time a player would receive priority, then the rules would simply state this same information at a different time... in other words, not sure what's special about "when a player receives priority" that makes APNAP work.
    – GendoIkari
    Commented Jun 2, 2020 at 20:45
  • I don't think it's unheard of for their to be duplication in the Comprehensive Rules e.g. a chunk of 603 is duplicated in 117.2.
    – Philip Kendall
    Commented Jun 2, 2020 at 20:52
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    You may have misunderstood my comment. My point was that the reason triggered abilities that triggered simultaneously go on the stack in APNAP order is because the rules (603.3b) say so. So if the rules were written in the alternative way that the OP is asking about, where they go on the stack when they trigger, the rules could easily still specify that when multiple things trigger simultaneously that they go on the stack in APNAP order. Waiting until a player would receive priority doesn't seem relevant to that.
    – GendoIkari
    Commented Jun 2, 2020 at 20:55
  • This is not relevant to the question asked - If triggers went onto the stack immediately, rather than waiting until a player would receive priority, nothing stops them from being put on the stack exactly the same way they are now, in APNAP order. Nothing stops 603.3b from being written "If multiple abilities have triggered, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose." and changing nothing with regards to your answer.
    – Andrew
    Commented Jun 4, 2020 at 3:58

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