The interviewer (and the school) know that you're interested because you applied (and they gave you an interview).
Anything else beyond that is merely a sign of desperation via the use of a non-binding contract.
From the wise Med Ed: [What med schools…]
accept and desire are two different things. My institution, for instance, will accept practically anything a given applicant wants to forward along, but only rarely do we consider it a worthwhile addition to the package.
And yes, some of us have gotten a little jaded about LOI's. I could fill a barrel with all the post-interview correspondence I have received that has not translated into a single matriculant. This has all gotten mighty complicated and burdensome for what is essentially a zero sum game.
It's generally not burdensome for an applicant to upload something to the portal, and once in a great while it does tip us off with some useful info. I can think of one individual who had a stellar application, like Harvard/Yale/Stanford-worthy, and a superb interview, who sent us several updates and a LOI. We were somewhat perplexed by this person's tenacious interest in our program. Turns out there were family/geographical reasons behind the whole thing, the applicant just never felt comfortable directly playing that card.
When it comes down to waitlist time I will scan through what folks have uploaded post-interview. The vast majority of times it has no impact. Occasionally I have seen it hurt people's chances. Come to think of it, in my experience this is probably more likely, than such correspondence having a positive impact.
See
https://forums.studentdoctor.net/threads/loi-and-interviews.1252832/#post-18849958 for a classic example of why most Admissions deans treat these as lies.
