For me, it was a self-affirmation to let them know.
I disassociated due to my sexual orientation. I WANTED them to know that I saw nothing wrong with my it and that I was going to [essentially] remove them from part of my life before they could remove me from part of their lives. [They couldn't DF me because up to that time, I'd not acted on my sexual orientation.]
You're correct, that they don't need to know. In my case, I felt much better --- "out of the closet" --- by telling them why I was leaving.
I also told members of the congregation that were close to me [before I'd sent in the letter of dissassociation] why I was planning to disassociate. I didn't "give a rat's ass" about what those who were not close to me would hear. But, I wanted those who were dear to me to hear the truth and not have only the viscious JW rumor mill that was bound to start as the source of their information.