Letter of Interest. What to include?

[Rastis]

Hi,

I was recently placed on a waitlist for my top school and was told that one of the items that I can submit to strengthen the chances of being admitted is writing a letter of interest in the program. It might seem obvious or straightforward, but what items do you include in such a letter? Is it similar to a cover letter that you'd write for a job? Any advice would be greatly appreciated.

Thanks!