Everyone gets some sick satisfaction from overhearing someone talking smack about you, especially someone you've been suspicious about from the jump. When someone talks about you behind their back, they have all the power. However, when you overhear the sh*t talking, you get some of the power back and are at least able to defend yourself against their claims about you. This Uno Reverse Card puts the sh*t-talker in an awkward position because you were never really supposed to know what they felt about you. But now that you do know, they can't take it back.
These kinds of situations don't only occur in interpersonal contexts. They can also happen in the workplace. One Redditor shared the story of his wife's boss accidentally CC-ing her on an email discussing her termination from the company. She had no idea she was on the brink of getting fired, and Redditors shared their ideas about how she could respond to this news.