Q:

Why the different coarse threaded screws?

I'm seeing more and more of these screws (pictured below), which are almost triangular. I find I can only put them into Power Supplies and PCI(e) cards in cases, but they will break/strip away if I put them into a hard drive or a standoff for a motherboard

Notice the triangular shape on it? On the Root Access chat, I started asking, but no concrete answer yet. I don't assume it's a production flaw, as I've seen hundreds and replaced them with the "proper" round screws. It is coarse-threaded, not fine-threaded (i.e. for a DVD drive or floppy drive). What are they for, and why do we need them instead of the regular round ones?

A:

The screw has a triangular cross-section to the threads because that's a technique used for many "self-tapping" screws.  They are designed to be driven into an unthreaded hole and form a thread as they are driven -- the shape helps "form" the threads vs just cutting them.
They may occur with any thread pitch, but generally the shorter ones will have a relatively coarse thread, since they'd designed to be driven rapidly (with a power screwdriver on an assembly line) into sheet-metal.

A:

I've seen tons of these over the years building computers, they are often shipped as chassis screws (for panels, power supplies, expansion slot cover plates, etc.)
And you're right, using them in a hard drive can wreck the threads pretty well if it's slightly out of alignment when screwed in, but based on the ones I've seen supplied with computer chassis they are the right/same threading.
Basically they work well (enough) for sheet metal fastening but for softer metals like aluminum you're better off using more precise, round-shafted screws. 
The shaft shape is basically a Reuleaux triangle ('The shape, other than a circle, a manhole cover be made so that it cannot fall down through the hole'). 
As @sawdust points out in his comment, and now @DanielRHicks in his answer, they appear to be self-tapping screws intended for machine/assembly line usage. 
I've personally used them as a substitute, ghetto tap because I felt they we're cheap, we usually had buckets of screws around the shop, and I'd rather destroy them instead of better screws (which would be used to finally fasten whatever I was making the hole for). :)

