Is letting someone know you're unfollowing them on social media necessary?

So I've been noticing a trend (not just here but on various forms of social media) where people have felt the need to make either a post, comment, or private message some one announcing that they're unfollowing them and here's why. And I find myself utterly confused as to what are they trying to accomplish.

Case in point, someone who's art page I follow on Instagram made a fundraising campaign to help those affected by the recent attacks in New Zealand (originally had a $500 goal and have currently raised over $3,000.) This morning, she posted on her story a screenshot of a message that someone had sent to her saying that they would no longer be following her due to this fundraiser (I won't quote but needless to say, it was horribly racist.) And while she wasn't bothered by it, I couldn't help but wonder 'What was the point of even sending that message at all?'

I've always felt that everyone is entitled to their opinion (no matter how wrong it may be) but that sometimes opinions are better kept to themselves. And in the case of social media, if you don't like what you see, just press unfollow or block or delete the app and move on. So why announce to someone that you're unfollowing them?

Is it an attempt to get them to change their behavior or their opinion? Is it one last dig at a random stranger on the internet? And are there cases where it's appropriate to tell someone you won't be continuing to follow them on social media?