The idea is to help combat the echo-chamber effect, if a group goes toxic then rational people are likely to leave, further concentrating the negative users left in the conversation.
So the idea is that if there's a large exodus of people unsubscribing then that's valuable information if we can determine the cause. Hence if there's an option to explain why you're leaving it can help flag a group as in trouble, first to the moderators and then to the admins that "are these moderators doing their job".
Now the parties leaving could show their protest in comments or threads, but if they're already leaving then they've already forgone that option. I know on reddit I'm not going to try help a dieing subreddit, I'm just going to leave.