I don't think that it functions to "weed out" students, and there are certainly some students who think they want one thing (HY or P for the name, for example) but realize later that they got into Brown instead because the fit was there all along and they didn't even know there was a school that could fit them.
A lot of times, it takes knowing a student from a school to put it on your radar in a meaningful way. It does consistently surprise me how little students research colleges, given all of the information that is readily available.
When I was at Brown back in the dark ages, I was on the debate team and we regularly debated against kids from HYP and others. There were many students at other schools who would have been great at Brown, who didn't apply/go because they saw the school as a sort of New England Reed or older Hampshire. In truth, it can be either, but can be far from both, depending on your path.
There were also many students at Brown who probably would have been happier elsewhere, because they didn't realize that freedom equals choices, and living with the consequences of those choices, and they would have been more comfortable on a set path.
You really don't always know at 17 or 18 what you want or what you need. I think that the admissions committee is pretty good at sussing that out for prospective students.
While Brown has a confidentiality agreement with the interviewers that we are not to disclose the content and questions we are answering about applicants, I do feel I can answer this in the general sense. (Although I do think others may have actually posted those questions elsewhere on CC).
Brown does say that the interviews are extremely rarely a make or break part of the application. And I agree with Sakacar that there are students who are a great fit for Brown even though they haven't put Brown on their radar except as a "back up" to HYP, and often haven't found out much about Brown even by the time they have their interview. I think that is particularly true outside of NewEngland/NY and CA, where there is less exposure to broad out of state college selection. So, it's pretty common that a lot of my interviews will be about educating the student about Brown, and also why I give students a lot of slack for how little college counseling advice they have, particularly from most public schools around here. Brown does want to get a sense, if we can give them that, about how much a student knows about and is interested in Brown, and from other questions we answer about what sort of fit the student is for Brown. We give them more personal opinions, info, but that is just added to the "distillation" they get from other parts of the application, and obviously is based on a short session. Admissions also knows the value of that report will vary widely from one interviewer to another and has to take that into account too, along with the tendency of alumni to "root for", and gush positive over most of the students they will interview just by the nature of being alumni volunteers and the wonderful quality, of course, of all these applying students these days.
On a more serious note, do you, as an interviewer, make special note of those students who don't know much of anything about Brown at the time they are interviewed? I would think that Brown, with its special niche, would be eager to weed out candidates who think they are really HYP material and are only only applying to Brown as a "safety school."
Yes, as an interviewer I make special note of students who know little about Brown. For example, I've interviewed students who live within an easy drive of Brown yet never visited. And there have been applicants who had no idea what the open curriculum was (I kid you not).
However, admissions does NOT want us interviewers to figure out which kids would rather go to HYP than Brown (although many interviewers haven't gotten that message). The admissions office feels (very strongly) that if you've filled out an application and paid the fee, you really want to go to Brown. It could not care less if an applicant's first choice is Harvard or Stanford. I'm very serious about this -- admissions has accepted students who have made it clear that Harvard is their first choice.
Why? Here's one good reason -- when Harvard's admission rate is 6%, there is a really high possibility that Harvard will reject that kid.
And Brown is not a safety school for anyone (well, maybe for the .1% and celebrities).
As for the OP -- there are many ways to show that Brown is a good fit for you and your first choice, and applying ED is not necessary. In fact, Brown does not favor those deferred ED applicants in the RD round. Before writing your essays, spend a lot of time on the Brown website. Read deep into this forum, because in the last few years there have been dozens of threads that would give you valuable information to use in your application.
I've interviewed several applicants and have always noted when an applicant couldn't explain at all why they were interested in Brown apart from saying that it was a good school. I especially noted when an applicant didn't know anything about the "new" (open) curriculum, since that's a clear distinguishing feature of Brown.
I've interviewed for another Ivy for 20+ years in the Midwest. I've come across applicants who may have not known a critical feature of my school -- rare but not fully uncommon. Sometimes, despite this deficiency, I can still say "this kid would be a great match".
That coinicidence can occur -- and I've reported that on a few occasions.
T26E4 -- I agree, there are circumstances when an applicant does not know a lot of details about a college yet still be a viable candidate for admission. I would imagine that is an issue for inner city and rural students, particularly if they are first generation. It's my job as interviewer to suss that out.