According to this [MIT statement of purpose guideline](http://web.mit.edu/msrp/myMSRP/docs/Statement%20of%20purpose%20guidelines.pdf), the statement of purpose seems to be an important part of a PhD application.
However, I have also heard that, in the case of [MIT](http://web.mit.edu/), the statement of purpose is only reviewed after the application has been tacitly accepted. 

So, how crucial is this part of the application, and what part does it play in the process of graduate admissions at top schools in the US?