How Does Statement of Purpose Play a Role in Graduate Admission? I'm applying for MSc. in Computer Science.

The basic format for SOP would be (at least in the university which I'm applying to):


State your research interests and career goals
Outline your previous research experiences / contributions (if any)
Name the faculty members you would like to work with


Does competitiveness of research areas (that are stated in SOP) affect your chance of graduate admission? I'm talking about a master program, not a PhD. If two people are both applying for MSc. in Computer Science, does the applicant (who showed interest in less competitive field) have a better chance of getting accepted? I guess the answer is obvious for a PhD admission because the decision is made by professors, but graduate admission is decided by graduate committees.
I doubt that "competitiveness of research area" is much of a factor, even for doctoral admission. They are looking for candidates who have an appropriate background (mostly from the CV) and a good probability of success in the program (mostly from LoR). The first bullet in your list is always the critical part. But the second point should simply point to how your background situates you to achieve your goals. Naming a faculty member is important in some, but not all cases. More so in the EU (and UK) than in the US. 

But, the research area is important as a decision making issue in so much as there are faculty resources (including any named advisor) to support that area. If there are more resources in one area than in another, then candidates in the one with more is likely to be favored, at least a bit. So a school that doesn't do much serious work in AI, say, will probably look for candidates in other areas. This is especially true at the MS level, but also at the doctoral level. 

For faculty hiring, on the other hand, different rules may certainly apply.