why are there so many different types of words
Words reflect ideas, or actions. There are a lot of choices for these. And many people see the same thing differently, so use different words to describe it.
I'm not sure, but I think you might be asking why there are so "many different types of words" for the same concept.
If this is so, they may reflect small variations in the concept or different origins for the words. The Sapir-Whorf hypothesis deals with some of this.
http://en.wikipedia.org/wiki/Sapir-Whorf_hypothesis
http://www.aber.ac.uk/media/Documents/short/whorf.html
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I also searched under "language variation" to get these sources:
http://www.spectrum.uni-bielefeld.de/Classes/Summer96/textdesc/node8.html
http://www.unc.edu/~gerfen/Ling30Sp2002/sociolinguistics.html
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This may be more information than — or not the information — you wanted, but I hope it helps a little more. If not, respond to this post with more specific questions. Thanks for asking.