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.
hope this helps.