Well, in the Harry Potter novels, I think she all-caps the words for two reasons. One, to indicate that those words are shouted by the person saying them. Two, because the words are acronyms. The only one I can think of right now is SPEW, the Society for the Protection of Elvish Welfare. Oh yeah, and the OWLs and NEWTS (Ordinary Wizarding Levels and Nastily Exhausting Wizarding Tests).
It appears the language has changed in my lifetime as it pertains to acronyms (and even the meaning of the word acronym has changed - it originally ONLY meant a (pronouncable) WORD formed from the first letters of words - but things that were once considered only initialisms, because they didn't form words, such as FBI, are now considered acronyms).
I remember first hearing the word laser when I was in the 7th grade circa 1970. The librarian knew I was interested in science, and asked me if I had heard of a laser. I said no (reluctantly, as I wanted to always be up on the latest things, and was ashamed I hadn't yet heard of this new thing), and she told me it was a kind of light that melts metal. She then directed me to the latest encyclopedia yearbook with an article on the laser, which I promptly devoured.
But I distinctly remember the word LASER being in all-caps, specifically because it was an acronym (Light Amplification by Stimulated Emission of Radiation). This also may have been my introduction to the idea of acronyms, and they were always put in all-caps back then.
Acronyms are now written in lower case unless they indicate some proper noun (such as the FBI I mentioned above), and even that is probably just initialisms that are all-caps - I can't think of an example, but a proper or brand-name acronym that's a "real" pronouncable word only has the first letter capotalized, as with any other proper name). Thus nowadays the word laser is in lower case like any other word.
Here's the Wikipedia page on laser:
http://en.wikipedia.org/wiki/Laser
Scrolling down, there's an image of a handwritten page on the right where the word was coined, with the caption:
"The first page of Gordon Gould's laser notebook in which he coined the acronym LASER..."
I suspect this was changing or had already changed as of ten or fifteen years ago when/before the Harry Potter books were written, so for whatever reason Rowling made those acronyms all-caps, such usage looks odd to modern readers.
I recall reading in high school, perhaps in dictionaries, statements such as "this word means such-and-such, but a while back it meant someting different." Now I've lived long enough to actually see and notice this more and more in real life. Yeah, I'm getting old, but I find this stuff fascinating.