Company founder Pavel Durov noted in his public Telegram channel that the potential for misuse of old messages is getting worse. Something you said can be "taken out of context" and used to hurt you years down the line, Durov said. This ostensibly gives you "complete control" over conversations, rather than trusting someone else to use your chat history in a responsible fashion.
As TechCrunch warned, though, the feature could be as problematic as it is helpful. While it could be used to prevent revenge porn or similar incidents, a malicious user could quietly delete messages to create a bogus version of a conversation, or wipe out evidence of a crime. There's no consent here, or even notifications -- crooks could cover up their tracks, and abusers could use it to attack people while leaving no trace. Durov said Telegram was aware of the "potential misuse" and thought the control would ultimately be beneficial, but that might not be how it works in practice.