I am writing my personal experience. If your experience is different, please feel free to write on the comment section.
Writing Statement of Purpose is very important component of your application package. I have seen as your representative to the reader.
My experience in writing the SOP was pretty. I just wrote down whatever came through in my academic endeavor. There were few templates floating on the market about writing the SOP. If I followed the template by just changing names and places, I could easily get it. NO! My main idea was that I should write about my self and not be motivated by anything else (because I had seen other's). I started by collecting the relevant info. After writing it in straight-forward way, I looked over it. It contained my academic history; a plus was non academic activities. I think it was unique (actually true for eveybody) . If anyone decides to write their own, the following site is useful to get an idea of what others do write!
The site is useful to go through.
Take time and Enjoy!
This site gives some useful tips. As it says," SOP is the only place where you have the total control". What I like the most is that it gives some samples with reviews http://www.statementofpurpose.com/essay_samples.html