(Master's Degree) What are they 'precisely' looking for in a Statement of Purpose, and does a SOP matter when one has excellent GPA?
What I'll write is based on my experience with mathematics Ph.D. admissions in the U.S.  How far it generalizes beyond that may depend on the circumstances.  (I'd bet much of it would still apply to a research-based master's program in computer science, but perhaps not to an MBA program.)  Note that U.S. graduate programs in math primarily take students who have just graduated from undergraduate programs, so the target audience is not so different from those applying for master's degrees.


  I'm still not quite sure how to write a 'breathtaking' SOP to blow the reader's brain. 


It's impossible to write a breathtaking or superb statement of purpose, and I mean literally impossible: such a statement does not exist.  Even if Einstein and Shakespeare collaborated to write one for you, the best they could do would be to write a good statement of purpose.  Once your statement hits the ceiling, there's nothing more it can do to help your chances of admission, and the ceiling is low enough that a substantial fraction of applicants reach it.  If I had to make up a number based on my impressions from reading applications, I'd estimate that a third of all applicants write a good enough statement of purpose that trying to do better would be a complete waste of time, and many of the others at least write pretty good statements.  [By contrast, the ceiling is vastly higher for letters of recommendation, as well as for accomplishments as an undergraduate.]

It's certainly possible to write a terrible statement of purpose.  If you come across as a jerk or seem incompetent or dishonest, you can ruin your chances of admission, no matter how good the rest of your application is.  Even a statement that has nothing outrageously wrong with it can hurt your application if it's poorly done.

Unfortunately, there's a lot of bad advice on the internet.  Some of the common recommendations, such as childhood memories or inspirational quotes, will not help at all since they simply aren't relevant to the admissions decision.  At best, they come across as a little naive (and committee members will groan about another applicant who was inspired by Marie Curie), but they probably won't hurt your application if you include genuine content in your statement of purpose as well.  At worst, they will fill up your entire statement and render it useless.

Similarly, you don't want to write a "unique" statement of purpose.  It makes sense to include explanation if there's actually something unique about your application, but there's no value in writing a statement that's gratuitously different from everyone else's.  The admissions committee isn't trying to understand what makes you a unique and special snowflake, but rather what makes you a good prospect for graduate school.  That's not a genre of writing that generally benefits from wild creativity.

So what are committee members looking for?  This varies a little from person to person, of course, but here's a first approximation:


Do you know what you're getting yourself into?  Do you have a good feeling for what graduate school entails and what's involved in your chosen field?  Do you sound like you know what you're talking about?  (Don't bluff!  It's better to demonstrate an impressive undergraduate-level understanding than to look like you're pretending to have even deeper insight.)  There are any number of ways to mess this up.  You can sound clueless, you can display crackpot-style fantasies, you can sound like you are choosing grad school to put off having to get a job, etc.
Can you summarize and highlight aspects of your background in a way that shows some maturity and perspective?  This could include undergraduate research, an undergraduate thesis or capstone project, favorite courses, outside reading, etc.  Some students do things that sound impressive from a one-line description on the CV, but when they explain in more detail it sounds a little hollow, like they never really got into it or learned much from it.  Others are able to take similar experiences and fashion a more compelling case for going to graduate school.  For example, if you are describing undergraduate research, it's generally not enough just to give a brief descriptions of your results.  You should also explain the purpose and motivation behind the research and reflect on the experience.  (Maybe it pointed you towards what you'd like to do in grad school, maybe it showed you what you don't want to do, maybe it made it clear what you need to learn, etc.)
Can you explain anything that's unusual or worrisome about your background?  You don't want to overemphasize these things (your explanation should be just part of your statement, preferably not right at the beginning or end), but this is your chance to put everything in context and explain why any apparent flaws should not worry the committee.
Do you have compelling plans for the future?  You certainly don't need to have your whole career mapped out (that comes across as silly), or even your whole time in graduate school.  However, it's best to show that you are moving forward thoughtfully and sensibly, rather than just applying to grad school because that's what comes next.


You shouldn't panic about writing a statement of purpose.  Remember that your goal is not to write something brilliant, but rather something good enough.  (For example, there's no way you could demonstrate amazing plans for the future or profound insight you derived from reflecting on undergraduate research.  That just can't be done in a document of this length and style, so you just need to aim for good plans and good insight.)

It sounds like you are caught in tons of stress and planning before trying to write anything.  What I'd recommend is this:  Take a few hours and write a first draft, while keeping in mind the questions I listed above.  Then get feedback from friends and mentors, and pay the most attention to feedback from people who have served on admissions committees.  Probably you'll decide to make some minor changes and be done with it.  The worst case scenario is that you somehow did a lousy job, in which case you should reflect on what people didn't like about your first attempt and try again.  Either way, you'll have a decent statement of purpose by your second attempt, and probably the first.


  Does the Statement of Purpose matter as much if one has a GPA of 92%? 


Some aspects (such as explaining poor grades) may be less relevant, but most of the things I listed above still matter.


  My interest in my field of study just happened over time; one day I saw that, hey this is pretty a fascinating filed of study and it totally matches my interest in different fields, I can apply the knowledge gained from this field to almost anything. So nothing extraordinary, it just happened with no reason.


You're right that this is nothing extraordinary, but that's fine, and it's very far from having no reason at all.  A bad statement of purpose says "I've loved area X since I was a child and have always dreamt of contributing to this field.  Admission to the University of Y will help me achieve my dreams."  That's nothing but a fancy way of saying you'd like to do X, without giving much of a reason.  Instead, a good statement of purpose could say "Area X fascinates me because it brings together my interests in A, B, and C.  [...further explanation...]  Among the many applications, D and E particularly appeal to me.  [...further explanation...]  Faculty members U and V work on related topics, which is why the University of Y would be an excellent fit for my interests."

If you can elaborate on your reasons that are "nothing extraordinary", you'll already be well on your way to writing a good statement of purpose.