whether or not these letters help is largely circumstantial and depends on a lot of things, for example the school, whether a decision regarding you has already been made, whether the letter gets put into your file by the admissions staff, whether someone actually reads the letter, whether the reader is awake when he reads it, etc. you write and send these letters without knowing if they will help. if the letter is well-written, it is improbable that it will hurt you. hopefully most schools have a methodical way of keeping track of such letters.
i would send the letters as soon as possible, in the chance that it has a positive impact on your outcome. if you are waitlisted, you could send another one. see past posts for more regarding the topic.
I wrote a lot of them, so its my belief that they can only help your app...if you're in a post-interview situation, def send out update letters as soon as you have something to update the school with...send a new update letter everytime you have something worth mentioning to the school that will help your app...hopefully these letters do get read by the admissions director or dean, and do get placed in your file, and when they do they will hopefully help when the adcom reviews your app...the more correspondence you have with the dean or the adcoms, the more they'll recognize your name when reviewing your app, and they'll know you are really interested in their school....so you should def get them out asap if you have an update! Good luck