Why should I buy a 50mm prime lens?

Challenge #5 in the Opportunities series. Hosted by juanlucas.
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I made a mistake... haven't bought a 50mm prime lens yet. Show me why I should buy it!! MANDATORY to indicate brand/model of lens used for taking the picture (of course, only 50mm prime lens for this challenge).
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The boy
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Camera:
Lens:
Submitted: Friday, 1st July, 2011 20:09 (GMT)
Taken: Thursday, 30th June, 2011
Focal length: 50 mm
Shutter speed: 1/20 sec
Aperture: F2.8
ISO: 400
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