Not sure but some ideas from this on the net:
[i]All caps are typically used for either of two reasons:
Capital letters are often used on covers of magazines, in logos and artsy-typography, usually to emphasise the visual style of the letters themselves, rather than the word. (Example Image)
Capital letters can be considered a third form of emphasis, among Italics and Bold text. They are used to denote a louder, almost shouting (and in many cases actually shouting) pronunciation.
This is done by tabloid newspapers, for example:
Political Correctness Gone MAD!
(in this example, the exclamation mark is used, erroneously, to add even more volume)
Legal documents sometimes use capital letters to denote areas of special importance. They are, presumably, trying to avoid the area being overlooked by the reader, by making it stand out from the rest of the text.
This product comes with NO WARRANTY.
There are also technical reasons for using capital letters. Here are just some examples of this:
Typesetting systems (old computers, stencils, …) that don’t allow for italics or bold text may encourage some to use capital letters instead
Capital letters to denote a SURNAME as opposed to a first name (according to wikipedia, this is done in francophone countries)
There are suggestions, some call it a consensus, that upper case text is harder to read than correctly capitalised text. This, to my knowledge, first manifested itself in public policy when British road signs were changed in 1957. The Worboys Committee proposed a new type face called Transport, using lower case letters for the first time. This is based on the idea that one reads the shape of the word rather than each individual letter. The All caps article on Wikipedia summarises the evidence for this.
It’s not even always a good idea to capitalise acronyms and initialisms, as illustrated by this ridiculous example:
Have they turned on the LASER yet?
Rhodri rightly says that this is uncommon and only happens to de-facto words.
As Jimi pointed out, ALL CAPS in email and instant messaging conversations is considered shouting, and people who use it extensively are considered trolls.[/i]
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