What's the use of it?
It is used if HTML is disabled. You can style text without using HTML but you can get the same result.
See the examples below:
Smilies:

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URL & Mail:
[URL] http://www.google.com [/URL]
[EMAIL] mueller@web.de [/EMAIL]


Font:
URL & Mail:
[I] italic [/I]
[U] underlined [/U]
[B] bold [/B]
[MARQUEE] marquee [/MARQUEE]


Images:
[IMG] http://www.extreme.de/bild.gif [/IMG]



Smoovote:
[SMOOVOTE] Umfrage ID [/SMOOVOTE]
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