What are traps for -- specifically, traps in the dungeon? Why are they there from a gameplay standpoint? What play purpose do they serve? And given all that, how do you implement them?
For me, there are 2 kinds of traps: attrition traps, and puzzle traps. Attrition traps are things like spiked pits and poison needles and crossbows behind the door. They are there for the same reason "easy" fights are there: to whittle down the resources of the PCs so the player shave to make meaningful choices about how much farther to push to reach their goal or collect some treasure or whatever. And while I try and make sure these sorts of traps make sense in the context of the dungeon at hand, I don't overly worry about "why haven't the wandering monsters tripped this yet?"
Puzzle traps are full encounters, with multiple stages of both danger and disarming: the room slowly filling with poison gas after the doors lock, the descending spiked ceiling with no apparent way out, the room with oscillating reverse gravity and arcing lightning bolts, etc... These usually occur in weird places with a funhouse quality and I either do not worry about how they got there, or I come up with some completely bonkers justification.
What purpose do traps serve in your games? How do you implement them? How do you feel about traps as a player?
I mean, from a psychology perspective, traps are for surprise and threat. They are for the reaction you get when the dice break out after the DM says "Okay, everyone roll a DEX save." Uncertainty! Attention! Compelling action and threat!
For me, the kind of elaborate traps that take up an entire encounter can be kind of unsatisfying in the way that puzzles often are. Roll a bunch of skill checks or guess what the DM was getting at (better hope they didn't leave something out!), and you know that the DM isn't just going to kill the party with poison gas, not when they have a whole rest of the adventure out there, so it doesn't feel like the risk is there. The idea of them is often cool, and they make a good companion to a combat encounter (where you need to make tactical decisions about using your actions to disarm the trap or stop the skeletons that are attacking you that don't breathe and so aren't affected), but on their own, meh.
Traps are often, IMXP, best placed as jumpscares. You can usually kill one character (they're one healing potion away from getting right back up), you can force the party to react to the sudden change. And yeah, the tension of "can we actually complete this dungeon?" ratchets up a notch as now you have one less long-term resource to spend. This encourages a sort of rapid-fire trap that can be encapsulated in one saving throw. And the idea usually isn't to avoid them, but to make the party spend resources if they want to avoid them (a spell slot spent on healing or a spell slot spent on a divination spell to find out which path is trapped or whatever). That adds to uncertainty and tension to the exploration.
And though modern monsters are pretty sanitized for an expected combat duration, I'd say that monsters often fill the role of traps, too. A mimic, for instance, shouldn't be a fair fight, it should be "suddenly, Lidda is devoured." The combat is just one way of rolling a bunch of dice to quickly resolve it.
And as I've discussed in the Exploration thread, cliffs and weather and rivers and landslides are often "traps," too.
"Attrition" kind of makes it sound like the point of the trap is to whittle away resources, but the
point of the trap is to telegraph danger and uncertainty! You can't just assume that you're safe just because you didn't roll initiative! The dungeon is an active threat that is hurting you just by being in it. It's not safe here. Whittling away resources is one pretty effective way to communicate that danger mechanically, but it's kind of secondary to getting to the psychology of "this old ruin is going to cost us something every time we open a door."
