Why do admissions committees consider the Statement of Purpose to be important? Why do admissions committees consider the Statement of Purpose to be important? Anyone with a command of English should be able to write a Statement of Purpose, in principle, so it seems a poor way to compare applicants' research potential. If I were to judge an applicant (obviously I've never been in this position), I would much sooner at grades than at their Statement of Purpose, simply because grades cannot (in general) be 'faked'.
Why do admissions committees consider the Statement of Purpose to be important?


This question assumes they do consider it important, but how it's actually viewed varies enormously between departments.  In mine, it's not considered particularly important for most applicants.  If your statement is pretty good, then that's good enough (nobody was ever admitted by writing a great personal statement).  However, it can matter in special cases:


It gives you a chance to address anything non-standard or potentially worrisome about your application.  It can be really valuable for you to make a case for why these things are not in fact a problem.
It gives you a chance to shoot yourself in the foot.  For example, applicants occasionally sound like arrogant jerks, demonstrate that they like to talk about mathematics they don't understand at all (without admitting their lack of knowledge), or reveal that they have no idea how graduate school works.  The first two are major problems, and the third is worrisome.


It can also be helpful to learn things like the intended specialization or advisor, but we ask for that elsewhere on the application as well.

If you apply to a department that takes this approach, then you shouldn't stress out too much about the statement of purpose.  But of course other departments may do things differently, so you can't relax entirely.