I touched on the topic of falling to the ground, screaming in the fetal position in the post Walk It Out! There's more to "walking it out" after a tough set than the obvious physical reasons.
The practice of looking pained does (in my experience and opinion) make things feel worse than they really are. And practicing it on a regular basis makes you better at becoming a crumpled, heaving mess on the floor. Not exactly what we're training for, is it?
I know what you're thinking - 'They can't help it! They've pushed themselves to their absolute limit!' I can't respond to this other than to say that, in some cultures, the crumpling-to-the-floor-thing is much more prevalent than others, even if we were to control for task-difficulty and fitness levels. I believe most people would concede this if we pointed to specific examples...
So, if it is true that they can indeed "help it", then why on earth would anyone want to do this on purpose? Because most of us come from cultures where hard-work, regardless of outcome, is praised and rewarded, and perceived light effort (aka "slacking") is punished. Conditioning (operant, not fitness) takes place early for many of us in gym class or in sports practice sessions. Observable effort and exhaustion (or at least the appearance of exhaustion) gets you an "ATTA BOY!" and in the showers early. Taken to extremes, injury can be rewarded with compassion and a lighter work load. Most athletes can remember a time when they saw their injured teammate sitting on the sidelines and thought "Damn, here I am working my arse off... he's got it easy....". Looking unruffled might get you extra "smoker" set as a reward, added (unwanted) playing time when fatigued, - YAY!
In fact, there are really very good reasons to practice making something that's actually very difficult look like it's easy. If you believe that "perceived exertion" means anything at all (and some training templates are based on rate of perceived exhaustion scales so there is probably something to them), then it makes sense that you would want to do your very best to ingrain the habit of thinking that (and acting as if) 'it ain't no big deal.' Practicing this mind-set and reaction to strenuous physical exertion will make it easier to catch your breath and reset for further exertion if you choose to pursue them, or if they choose to pursue you... Just something to think about.