Well, I did this recently because someone in my family had been causing problems by policing my Facebook account and gossiping and spreading rumors about stuff that I had posted in the past and even some stuff that I didn't post.
The most recent offense caused a heated argument under my own roof and the person who started it wasn't even a Facebook friend any longer and my posts on that account were for friends eyes only so, then, how would she be able to see anything that I posted unless she were being fed info from another trouble maker.
On top of all of this most of my family has disowned me because I have disowned God and hardly anyone was acknowledging me on Facebook anyway under that account so, why not make a message letting people know that you are going to make changes to your account because of a troublesome person or persons and not knowing who you can trust and anyone who wishes to stay simply pm you or reply or react to a post that you made to show that they are still your friends?
I don't get why it is a problem for someone reading it. It's basically like giving a "heads up!" if you are truly my friend then, speak up so, I don't accidentally delete you, ok?
If they said nothing and deleted you then, wouldn't you wonder why and feel a bit upset that no explanation was given?