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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Pia at 50mm
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Camera:
Lens:
Submitted: Monday, 27th June, 2011 18:06 (GMT)
Taken: Monday, 22nd March, 2010
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Shutter speed:
Aperture:
ISO:
Notes: Minolta f1.7/50mm
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