I would add an exception to @RoboKaren's answer. Economics programs tend to place very little weight on statements of purpose. Econ is perhaps a bit different from other disciplines because the first year is all coursework.
In a statement of purpose for an econ program one should highlight relevant previous courses/skills (especially math/stats) and indicate a few areas of interest (e.g. "Macro labor" not a dissertation proposal). Minimize the fluff.
Several economists (including Susan Athey [Stanford] and Jeffrey Smith [Michigan]) have composed helpful guides on grad school admission. I'd bet others have done the same in other disciplines.
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The general point that I hoped to make with this post (which was implicit but which I will now make explicit) is that the importance of the personal statement (and the nature of the application in general) is particular to a given discipline.
Why don't economists care about personal statements? First, it is costless for applicants to overstate/misrepresent their interests, talents, background, enthusiasm, etc. Second, given the other materials available to the adcom (see below), even a truthful statement is more or less superfluous. Third, deemphasizing the personal statement reduces the advantage enjoyed by native English speakers in admissions.
What do economists care about, then? By far the most important component is faculty recommendation letters. It means something if a faculty member is willing to say that someone is "the best student they've had in eleventy years in the profession". Suppose a faculty member misstates the qualifications of the applicant and the applicant is admitted (and perhaps offered a stipend). If the applicant fails out, then (1) the adcom will be disinclined from believing the recommender the next time around and (2) there may in certain cases be some informal social/professional consequences (i.e. "Hey remember that time you recommended that one kid and we wasted a spot and 25k on him just so that he could drop out in April of the first year?").
I don't think that this is a perfect system (or that admissions are perfectable) but that's just the way it works and has worked in pretty much any econ department for quite some time. Depending on your view of economics, it either more-or-less works OR goes towards explaining why econ is so messed up(!).