There is some level of self fulfillment in the undelivered letter.
When someone steals space in your head and there are a thousand things you want to say or explain, but the risk of saying them out loud out weighs the benefits. Perhaps the things you must say would take a relationship to a place that you are not ready for, or the things you have to say may damage other relationships that you are not willing to risk. It is difficult when you want to be open an honest all the time, but sometimes it just isn’t worth it. Then those thoughts or the anger behind those thoughts steal time in your head that could be better used for other things.
Write the letter, write the letter with every intent to send. Do not hold back on what you want to say or even worry about how it may sound. When the letter is complete you have a decision to make, to send or not to send. Maybe even the answer is tuck it in a drawer until the time comes to send.
The thing about words, once they are out of our heads and into others, they can not be taken back. So, write the letter, get them out of your head and then later decided when it is time to hit the send button (because let’s face it- who uses paper) or when if it becomes time hit the delete or rip away. The self fulfillment in the undelivered letter is to get the thoughts out of your head and for some reason that sometimes is enough. Just knowing that you could put it into words sometimes is better than the delivery.