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:

:arrow:
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:badgrin:
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:D
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:confused:
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:cool:
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:(
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:doubt:
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:evil:
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:!:
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:idea:
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:lol:
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:mad:
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:neutral:
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:question:
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:razz:
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:oops:
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:roll:
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:-(
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:shock:
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:)
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:-o
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;-)
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:-*
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:tear:
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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]


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


Youtube Videos:
[YOUTUBE] Youtube video ID (11-stellig) [/YOUTUBE]


Smoovote:
[SMOOVOTE] Umfrage ID [/SMOOVOTE]
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