It does show that you have researched the curriculum, and are independently directed. (although don't call them majors, use "concentration" from now on!).
I think it also helps some to have a less popular field of study. However this group of concentrations might still be labelled as "likely premed" applicant.
Brown is also sure to know that some applicants "scam" the process by professing to plan on a lesser followed concentration, fully intending to be premed etc. (Your HS activities do help reinforce your intended concentration in your case.) Conversely it is also known that a fair amount of "premeds" etc, will decide that isn't for them and switch to something else once they are in University. (and hit Orgo.) So, Brown does look for balance in the classes, but I think the essays about concentrations may be as much about hearing about how you organize your plans.