The Why: Pits

Pioneer Footsteps 
The classic 10' deep pit... I'm no historian, but I'd wager the pit was one of the very first dungeon traps. Gary probably used a million of them. They were so ubiquitous - every dungeon key in nearly every dungeon ever designed (both in the professional and the "bored in Math class" style) would have an entry for pits. Pits were expected, as much as in the days of blue grids and typewritten addendum as they are now.

There's something wholly nostalgic about encountering the first pit of the dungeon - the thief sizes up the jump, the cleric starts measuring rope, and the fighter ponders dropping things into the darkness. It's a scenario we may not all have experienced in exactly the same way, but it's something we all inherently understand.

By the same virtue, there's a similar connection one makes to the players of the past when their party comes upon a pit; a shared universal experience of the game. When you stand upon the edge of a pit, you stand in the footsteps of a thousand characters before you, going all the way back to the start of the hobby. Groups of friends, gathered together in game, all pondering "how should we approach this pit situation?".

How did Gary Gygax describe his first pit? What about Dave Arneson? How did Rob Kuntz get past his first one? Or Monte Cook, or Ed Greenwood, or Chris Perkins?  We may never truly know. They've all done it, most assuredly, and in doing so perhaps we all hold a small bit of the same feelings, frustrations, and ideas that touched their minds when they came upon their own pits. In that sense, we've all gone through the same sensation, we've all been there. We've all paused at the pit for a moment or two.

The Plausibility Factor
There's not much complexity to the common dungeon pit. It's design - literally a hole - is a concept even a brain-damaged goblin could grasp. It's a trap one could build with their own bare hands, had they the time for it.

Importantly, a pit can take many different simplistic forms; easily recognized for what they are, but not so out of place wherever they're found. A muddy sinkhole, an open manhole, an empty mine shaft, a patch of termite-eaten floorboards... just "re-skinned" pits. Yes, there is a touch of imaginative effort required to ensure a thematic match (or "the right pit for the right place"), but DMs will rarely find themselves needing to justify the existence of a pit trap - as a design concept, almost any kind of pit can reasonably be encountered in almost any kind of environment.

Even the most outlandish of pits in the most implausible of places can be spun into a pertinent fixture with a believable re-skin. A lava tube jutting up into the castle stables could be spun as a probe from the Magma Empire. A punji stick trap at the bottom of the sea might be skinned as a breeding bed of Firecone snails. Loose flagstone in the arctic snow might well become the thin-tiled rooftop of some ice-buried laboratory. And so on, and so forth.

Spectrum of Danger
A pit can be filled in, jumped over, or climbed out of. At it's base design, it's hardly an impediment at all, especially considering the pit is a benign trap (as opposed to a more pro-active trap, one which actively moves to harm, like a falling stone or poison dart gun). A basic pit trap can be made less and less dangerous with such simple changes as reducing its depth or making it visibly obvious.

However, as simple a concept as the pit trap may be, it can also paradoxically become one of the most difficult traps for players to overcome merely by incorporating a touch of creativity. Rain from above, jab from below, pivot the walls, entomb in the muck, spawn horrors, lock the cage, flood everything, crush everyone. The pit trap is eminently customizeable. DMs learn fast that a straight fall just doesn't excite players after the first few encounters, and as such, pits become a veritable breeding ground for sadistic trap ideas (as Grimtooth would certainly attest).