They're prudent for things which require little torque and break when too tight, most notably screws that clamp hollow carbon fiber parts like seat posts, handlebars, and fork steerers.

They're a good idea for things which need to be tight enough many people need some encouragement to pull harder. Cassette lock rings, crank bolts, finicky saddle clamps, etc.

Crank arms (even most splined ones) have tapers, and if you don't get them on tight enough they'll wobble, the hole will open up, and you won't be able to make them tight enough to stay on until you buy replacements.

Some saddle clamps are fickle and slip when not torqued disproportionally tight with assembly paste only helping a little. Some snap bolts perhaps due to torque. Having finished too many rides with my saddle in an uncomfortable position or detached I switched back to more reliable seatposts (Campagnolo C-Record posts stay put with a normal tug on a 6mm hex key, have no teeth limiting angle, and just work).