Being fatigued makes you less effective at various tasks, both physical and mental. Not all games have a fatigue tracking system, but for those that do the effects can vary from a small skill penalty for low levels, to being almost completely useless and about to fall asleep at any second. Naturally, it would be dangerous to engage in certain activities while heavily fatigued, and not just limited to operating vehicles.
Fighting off an orc ambush in the middle of the night when most of the part have only had a couple of hours of sleep can be dangerous enough, but if you assess penalties due to tiredness it can be even more challenging. Although in this case one could assume that adrenaline will kick in and cause the defenders to fight effectively.
For the first wave of orcs...
Another good thing about a fatigue penalty system is that the penalties are often generic enough to also be useful for the effects of intoxication, with alcohol or other drugs. You can easily just combine fatigue and intoxication by adding levels together and assessing penalties for the combined level.
Because what party of adventurers wouldn't end up tired and drunk at the same time when orcs ambush?
Commentary by Keybounce (who has not seen the movie)
"You flying would be like riding a unicycle over a lake of boiling lava"
May I comment on the impossibility of a lake of open air boiling lava?
And, we see the real effect of Fatigue in this system. Rey can't fly until she gets it taken care of. That's actually a potentially serious disadvantage.
Or... actually, all she says is it's not safe. Since when do adventures worry about what's safe? Doing dangerous risky things is how you gain experience, levels, and power. Wait, Pete is turning down a chance for power?
Commentary by memnarch (who has not seen the movie)
I wonder how much not getting tired is another benefit of playing a mechanical character. Sure they don't need to sleep, but there's probably some downsides that come with it.
People ride bikes without helmets all the time, you'll be fine. Never mind that it's quite the risk and getting into a crash would be like sand-blasting yourself. And either the default Piloting is very very bad, or Rey's hoverbike takes a crazy amount of skill to safely use. That would be in character for Pete to have a dangerous ride that only he could use safely. Which makes the lack of hoverbike security have more some sense now; nobody would try to steal it for fear of killing themselves. Then you've only got to keep the salvagers from tearing into it when you're not around.
Unless they're lava-breathing crocodiles, I don't think they'll last very long. Pete should know though that if you can get a helmet to be made out of boiling lava, it would be totally awesome! Clearly you're dealing with a magical helm of some kind at that point otherwise the lava would cool off pretty quickly, so there needs to be something keeping it both hot and in helmet form. And once you've got both of those things, it'd be trivial to make it safe to wear.
Rey: Well, it can wait a few hours while I get some sleep.
BB-8: No! We have to go now!
Rey: I have a Fatigue point. It’s not safe to fly.
BB-8: I can fly. I don’t get tired.
Rey: Oh? And what’s your Piloting skill?
BB-8: Um... Default?
Rey: So. Me flying would be like riding a bike without a helmet. You flying would be like riding a unicycle on a tightrope over a crocodile-infested lake of boiling lava.
BB-8: Okay, okay, I’ll hang out with you until morning.
Rey: And your helmet is also made of boiling lava.