Likely high temperatures and/or chemical resistance, maybe very high tightening torques or force, but possibly others, too. Mostly more specialized fastening, I would guess.

But think about the bolts and nuts that hold the exhaust system to a car engine. Temperatures are going to be very high and nylon wouldn't withstand those temperatures. Things like that.

Lock washers?

According to a NASA fastener design manual[1](page 9) split-ring style lock washers are useless for locking purposes. They say that star washers do proved some additional friction/locking action, but at the cost of damage to the surface(the friction comes from digging into the surface).

[1] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1990000...

Thanks. That's great to know. And lots of other useful information.

Fretting. Under high lateral force and vibration, I get fretting and I hate it.