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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Fall Colors. SMC Pentax-M 1:1.7 50mm
by rjpinaz
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Camera:
Lens:
Submitted: Monday, 27th June, 2011 07:16 (GMT)
Taken: Sunday, 31st October, 2010
Focal length: 50 mm
Shutter speed: 1/2000 sec
Aperture: F1.7
ISO: 200
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