Basically the point of a LOCI is to convey your interest in the school. You can talk about what about the school makes you think it is a good fit for you. Also try to convey that they are your first choice, if they are, without sounding like you are begging. IMO, a LOCI won't have a huge affect on your decision one way or the other. But if schools take candidates off the waitlist, I'm sure they would rather it be someone that they feel really wants to attend. Try to convey that in your letter.