Drain resources. Hit points mainly.
Occasionally pose a lethal threat (spikes, long falls, sheer walls etc).
Because people can fall in, injuring themselves, and possibly become trapped.
So, the same thing mine do then???
Even a 50' pit is easily climbed out of with a rope which all adventurers have. Unless you're doing something radically different to me, climbing out of a pit using a rope with a wall to brace on is 'DC dont bother rolling, you make it.'
Sure. Getting out of a pit isn't hard
IF you have help, a rope (and people to lower it to you or a grappling hook), easy handholds (rock projections and crevices, roots, etc.) to climb. But a lot of people can't climb ropes easily even (remember how many kids, even older teens, had issues with rope climbing in gym class?). With a rope, you gain advantage on your check for climbing and the DC is usually lower anyway (typically 8 or so) IMO.
Consider this example: even a DC 10 with rope and +0 modifier will only
fall 4% of the time. If you have a +4 or better modifier, you would only
fall with double 1's, so 1 in 400 chance. With 4 checks, the +0 modifier would fall (at some point) about 15%, but with the +4 or better it is only 1%.
So, yes, with help, and even a bit of ability or proficiency, it isn't
hard to get out of a pit. Without help, it is much harder but still very possible. That's all I've been saying since the beginning. But all this started with us talking about climbing a cliff by oneself.
Anyway, the big difference (as I already noted) is in game style--if you want to take the time to play these obstacles out and make them part of the PCs experiences.
Not result in Achilles comically struggling to get out of one for 5 minutes of game play..?
Given his strength and proficiency in Athletics, it shouldn't be a problem.

So, I really don't know why you would bring it up unless your Achilles is a wimp?
shrug