D&D 5E - Take the Ready action.

If X moves towards me, I dash away.

If X moves adjacent to me, I disengage.

You might want to wait if you're in a tense negotiation. Standing there's bobbing and weaving while negotiating is kinda indicative that you expect to be attacked. So you ready a dodge.

You can't help a person do a thing until they try to do that thing. I can't help you pick a lock until you try to pick the lock. "When X tries to pick the lock, I'll help" is a readied help action. If you're in combat, that precision matters.

If you're poking around a room you're not hiding. If you're not supposed to be there and a guard shows up, ready action to hide.

You aren't going to search a fighting goblin. You're going to search a dead one. If you're in combat, that precision matters.

Basically, it's all about the pressure and precision of timing things in combat. Hence "actions in combat". Initiative order can matter a lot. So certain readied actions can matter a lot.

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